Identify It! Answer for 10-13-2010

Thanks to everyone that played this week’s Identify It! Challenge. For this week’s challenge we asked fans to guess the informal name, scientific name, give several facts, and identify one unique feature of this specimen of spider. Here is the original photograph:

 

Writing Spider : Argiope aurantia

The answers are:

Informal Name: Writing Spider (AKA Black and Yellow Garden Spider)

Scientific Name: Argiope aurantia

Facts:

• This spider is called a writing spider because it often creates zig-zags (that look like the letter Z) or X’s or other patterns that look like letters. It does this because it often weaves very large webs and the patterns make the web visible to birds, squirrels, and other animals that might knock its web down.
• Although incorrectly drawn in the cartoon, Charlotte from “Charlotte’s Web” was a writing spider!
• This spider will bite human beings if provoked but is not aggressive and its venom is usually not very potent. (its still a good idea to leave all spiders alone though)
• Argiope aurantia often leave an egg sack with thousands of eggs at the end of the fall season.
• The size and complexity of its web are a good indication of how well fed it is.
• Males are small and uninteresting marked, often building a small web adjacent to their female mates. Females are much larger (on the order of 3 inches in diameter) and are brightly marked with yellow spots and patterns on their abdomen and back.

Unique to this specimen: If you look closely at the photograph you will notice that this spider only has seven legs. It isn’t clear if the spider lost its leg at some point or if it was born without it.

This specimen has lived for at least the last three years on the side of the Sylvan Learning Center in Burlington NC. (As one fan pointed out its interesting that a “writing spider” would take up residence at a learning center.)

More Higher Resolution photographs can be found at the following link:

Click Here for more High Resolution Photographs

or also at our “photo” tab at our fan page on facebook (http://www.facebook.com/HarrisEducational)

 

Belly View (through the web)

3/4 Side View in Web

Thanks to everyone for playing this week’s Identify It! Challenge. Stay tuned for the next one!

 

 

Identify It! Answer for 6-3-2010

This Identify It! Challenge was a little different than normal. Thanks to everyone who DIDN’T post an answer! I knew when I posted this weeks picture that a portion of our audience would likely know exactly what this object was. Imagine my delight when one fan posted a picture of himself with his very own!

Heathkit Model 336 High Voltage Probe

This item is a High Voltage probe manufactured by the Heathkit corporation as a build-your-own kit in the early 1950′s. In fact its is a model 336 “Television Test Probe Kit.” It is meant to work with multiple models of their popular vacuum tube volt volt meters. The unit pictured here was bought along with a model V5 Heathkit VTVM kit and originally sold for $5.50 in 1951! It extends the range of the VTVM so that it is possible to measure voltages up to and including 30KV.

What Its For:

This probe is meant primarily for use in servicing cathode-ray-tube based television sets. A CRT works like any other vacuum tube in that a filament heats a cathode which then emits electrons (a.k.a. the Edison effect). Since electrons are negatively charged they will be attracted to, and accelerate towards a positively charged plate. In an ordinary audio or radio tube the plate voltage is anywhere from a few hundred to 450 volts. Since a CRT is much larger it requires a much higher positive voltage in order to operate. A black and white TV might have a voltage from 7KV to 20KV depending on the size of the tube and a color TV (with its three electron guns) operates at anywhere from 20KV to 30KV. This high potential accelerates the electrons in a beam so that they can strike phosphorus on the screen which then emits the light you see. One problem, especially with older TV sets and early color sets is that if the current flowing through the tube is too high the picture tube (and other tubes relating to the generation and rectification of the high voltage) can emit X-rays. It is important to be able to measure the high voltage (in the KV range) present at the anode of a CRT in order to adjust the high voltage circuits to eliminate the chances of producing X-rays and also provide enough voltage for a bright focused picture.

How It Works:

The probe body is made of plastic that while clean and dry will not conduct electricity. The fins around the base act as a hand guard and also increase the surface area of the exterior of the probe so that a spark can only jump from one end of the probe to the other if the voltage is well in excess of 30K volts. Similar designs are used on glass insulators on telephone and antenna terminals to help dissipate the potential for lightning damage.

Inside the probe are two specially designed high-voltage / high-frequency resistors which are bolted together from end to end forming a path for electricity from the tip back to the base. These resistors are made of a carbon stripe which is deposited in a helix around a ceramic core. The value of the two resistors in series is 1090 megaohms. The entire resistor is then glazed over to seal it from moisture and insulate it electrically. Since the CRT anode voltage in a TV is pulsating at a rate of 15.624 kHz (for NTSC TV’s in the US) there is also some capacitive and inductive reactance which is accounted for in the resistors design.

The probe is designed to plug into the phono-jack style connector used on Heathkit meters of this period. The resistors inside the probe add to the overall impedance (resistance to alternating or pulsating current, i.e. A.C. Resistance) of the VTVM extending its own input impedance of 11 Mega-Ohms many times. This has the effect of adding a 100x multiplier when using the 300 Volt scale on the meter.

And More:

Since most modern electronics run on lower voltages and since CRT’s are becoming more and more rare in new electronic devices this probe is largely obsolete. However it is still useful for working on older TV’s and I have also used it in some high voltage experiments (along with a high-voltage Diode to rectify the alternating input source to D.C. before measuring.

You can learn more about Heathkit at the following links:

The Heathkit Virtual Museum: http://www.heathkit-museum.com/

Heathkit Schematics: http://www.vintage-radio.info/heathkit/

Manuals and Pictures of Heathkit Test Equipment: http://www.nostalgickitscentral.com/heath/products/test.html

A cool site about Vacuum Tube Volt Meters: http://oak.cats.ohiou.edu/~postr/bapix/VTVMs.htm

Thanks to everyone who took part (and especially to those who avoided taking part) in this weeks Identify It! Challenge.  Stay tuned for more Identify It!’s and feel free to contact us and share your own images related to science, technology, engineering, or mathematics that others can guess about and learn from.

Identify It! Answer for 5-27-2010

This week’s Identify It Challenge was guessed pretty quickly, and one fan even found a picture (until this post the only picture I know of this device on the internet today!) So great work everyone.  I took the liberty of obscuring the IBM logo on the original picture when it was posted in order to make it a little harder to look up the item. Here is the object without the IBM logo obscured.

The IBM Magnabelt Executary Model 224 dictation recording machine (1960s)

This item is an IBM Magnabelt Dictation Machine (model number 224) also sometimes called an “executary”. IBM first introduced these machines in the early 1960′s. It was used to record dictation in the office and also as a method to record audio in the field by reporters. These machines are fully transistorized which lends to their smaller size and ability to be operated by battery. They did suffer from sound quality issues partly due to tracking problems with the recording belt, but also due to the poor quality of the included microphone. Today’s microphone technology is light years ahead of what was possible in 1960. In spite of these problems these units were a commercial success.

Magnabelt from another Angle

Closeup of the Microphone Connector, Proprietary Connector, and Microphone

As you might know Thomas Edison invented the record player. He first marketed the record player as a business tool to record dictation. Early models used wax cylinders. As time went on other methods to record sound were devised including wire recorders and eventually tape recorders. The “executary” uses “magnabelt” technology where audio is recorded as a helical stripe along an endless magnetic recording belt. The belt is a mylar plastic belt with ferromagnetic material deposited on its recording surface.

Executary with leather carry case removed

The unit could be used in the field on battery power, or in the office connected to a base station via a proprietary power, audio, and control cable. For use in the field the unit had a thick leather carry case (which was the style at the time for portable electronic equipment like transistor radios). The leather case is held to the unit with a large captive thumbscrew. The picture above shows the unit with its leather case removed. Its exterior is a thin steel shell case with a flat gray paint job. The end piece is a black plastic.

Executary Case Slid Open

To gain access to the magnabelt recording media or in order to adjust the recording level (a curious design limitation given there is a recording level meter in the operating controls) you can press a metal button on the back of the case and simultaneously pull the actual mechanism out of the case. There is a stop inside the case that keeps you from removing the entire case.

Recording Head lifted Open

To get to the magnabelt you must first release a catch on the recording head mechanism and it swings open to about 25 degrees.

Magnabelt being removed

Closeup of Mechanism with belt removed

You can then gently lift up the portion of the belt that is pressed down by the recording mechanism and slip it off of its feed rollers. One benefit of the magnabelt over other kinds of tape (reel-to-reel, cassette, or cartridge types) is that it can be folded flat and sent through the mail to another person with another machine at normal postal expense, taking up much less space. One drawback to the technology though is that the physical alignment and tracking on a magnabelt is lost once it is removed or replaced, requiring the use of indexing and pitch controls.  Getting a totally accurate reproduction of the sound is more difficult.

Detail of Recording Head

The “executary” used a single head for recording and playback and does not include an erase head. Recording over used magnabelts would be possible but you would have to have used an external degaussing machine to scramble the magnetic domains on the tape to erase it first otherwise you would likely have had a mixture of old and new sound recorded. (especially if the belt had been removed and replaced… due to the tracking problem as mentioned above). The head is visible in the photograph above. The head is moved from left to right by a rotating threaded rod, this rod is geared to the motion of the magnabelt’s drive by a toothed belt. As the mangabelt moved below the head the head slowly moved to the left thus recording a helix around the belt. There is a tone/tracking adjustment wheel on the body of the recording mechanism (indicated with a tuning fork symbol). This adjustment moves the head slightly to the left or right in relation to the screw mechanism so that you can find the track if the belt is moved or if a belt from another machine is being played back.

Battery removed from the unit

The “executary” was powered by a proprietary 10.7 volt non-rechargeable battery that is marked with the IBM logo but manufactured for IBM by the Mallory corporation.

Typical use position

The unit was typically used by holding in the right hand and using the thumb to operate the controls. Inside the recessed blue square are the following controls and indicators: A tiny recording level meter, the power and playback volume control knob, the record button (which can be used as on/off or locked on for continuous recording), and a manual tape advance control lever that starts the tape moving for playback or recording. Also on the unit is the tone/tracking control, an indexing control that in some way marks the index strip of paper to indicate a start/stop/edit point and a manual release that disengages the tape head mechanism from its drive screw so that you can advance the listening/recording position on the tape. This is all relative from 0 to 100 on an index strip of paper that is held in place by friction inside the tape record head mechanism. This strip would be kept with the magnabelt if removed and used to label it.

Closeup of User Controls

In the later 1970′s advances in micro-cassette recording technology, integrated circuits, and better microphone technology replaced the “executary” style magnabelt recorders for office and portable use. Today micro-cassettes are still used but are largely being replaced by tape-less technologies such as recording directly to flash memory. In fact most cell phones have a recording capability for audio notes!

To learn more about the Magnabelt “Executary” and other older and proprietary audio recording technology check out this website: http://www.videointerchange.com/audio_history.htm

Check back again in the future for more Identify It challenges and their answers.

Information About Microscopes for Home Use

A friend who is a parent recently asked me for advice on how to pick a good microscope for use at home for his children. (I think some lucky kids might be getting an interesting Christmas present this year) I thought I’d publish my thoughts here so that more parents might benefit. I’m not going to endorse any specific brand or reseller but instead will try to give you some information that will help you make informed choices if you are also in the market for an educational microscope.

First a disclaimer. I’m not a biologist or a biology teacher and so I don’t have years of experience first hand with microscopes. I have however had a few years of experience in the Educational supply industry and am aware of some of the issues you will run into when you are in the market for a microscope. Anyone with more experience should feel free to comment, make recommendations, or point out any issues I might have left out.

Buying a good microscope can be a dizzying proposition if you’d like to get it right. There are many manufacturers, resellers, types, and a huge price range from simple magnifiers all the way up to thousand dollar lab equipment. Microscopes and Magnifiers generally fall into price ranges including “Toy”, “Economy”, “Educational/School”, and “Industrial/Scientific”. In general you don’t want to waste your time or money on “Toy” Microscopes.

Types of Microscopes and their intended uses:

Magnifiers

• Purpose: Simple magnification
• Great For: Young kids to explore more details of the world around them.
• Neat things to look at: The back of a U.S. penny to see that Lincoln is sitting inside the Lincoln memorial, fabrics and threads, insects, textures on objects, wood grain.
• Magnification 5-30X
• Best Kind: Those mounted in frames used in the textiles industry for thread counting
• Worst Kind: Cheap Dime Store/Toy/Kids magnifiers
• Notes: Best to get some kind of frame and built-in light source (LED if possible). It is ok and even preferable to get a magnifying glass made out of acrylic plastic. If you can check the lens for distortions, light halos, or other aberrations.

Field/Pocket Microscopes

• Purpose: Higher magnification than a simple magnifying glass sometimes with features of an actual microscope but intended to be portable.
• Great For: Use outside or on a trip.
• Neat things to look at: Grains of sand or soil, insects, parts of plants, the surface of rocks and minerals.
• Magnification 5-150X (sometimes up to 300 in professional models)
• Best Kind: Those with higher quality optics, focus adjustments, and built in light sources (single hand-held and sometimes with a clip-on-frame for holding actual slides)
• Worst Kind: Units without a focus adjustment (toys or units that are really just magnifiers)
• Notes: If you have a child between the ages of 5 and 10, and don’t care to focus on very small objects (like cells, protozoa, etc) then a good pocket scope is an excellent and inexpensive choice to get a young mind thinking about the structure of the world around them. Some models do even have enough magnification to view large cells and structures in plants. If you have an older child or want to focus on very small objects a more traditional bench microscope (digital or optical) is probably a better choice.

Optical Microscopes

• Purpose: Viewing details of biological specimens, cells, and other very small objects.
• Great For: Science Fair experiments in biology, observing different kinds of cells, learning about biological concepts.
• Neat to look at: Prepared microscope slides, your own slides, cells.
• Magnification: 30-300X
• Best Kind: All metal construction with glass optics and built in light source (preferably LED since an LED doesn’t heat the sample as much as quartz or other high power white lights).
• Worst Kind: Plastic construction with plastic optics or mirror light source. The higher the magnification being used the more problems you will have focusing on a sample if the focus mechanism, stage, or light source is not sturdy and adjustable.
• Notes: Bench-top optical microscopes (just like the ones you used in Biology class in high school) are great for observing cells and other small objects. Keep in mind if you buy a microscope you’ll also want to have something to look at. The younger your child is the more important having a source of prepared slides can be. Many microscope kits include slides, slide preparation equipment, and pre-mounted slides of common biological specimens. Quality varies directly with price and if you go for a cheap scope you will also probably get cheap slide materials. Glass slides are better though might be dangerous for younger children. Never let your young child use this equipment without supervision. There are also great sources of prepared slides for use with any microscope. An excellent supplier is Carolina Biological Supply Company of Burlington NC.

Optical Stereo Microscopes

• Purpose: Viewing details on larger objects, especially rocks and minerals.
• Great For: More detailed viewing of rocks and minerals than can be achieved with a pocket or field scope, though not as good for tiny biological application due to the lower magnification involved.
• Neat to look at: Rocks and Minerals, Insects, other larger small objects
• Magnification: 10X to 100X
• Best Kind: Same rules as an optical microscope, all metal construction, fine focus control, good light source.
• Worst Kind: Plastic construction, plastic optics, indirect light sources.
• Notes: If your child has more interest in geology than biology then a stereo microscope may be a better choice since they can focus more on rocks and minerals. Stereo means two eye-pieces and thus the ability to have some depth perception (distorted through magnification) and so is better for seeing structures in 3D.

Hand Held Digital Microscopes

• Purpose: Viewing small details on objects like rocks and minerals, wood, circuit boards, etc. Similar to a field/pocket magnifier but tethered via USB cable to a computer and therefore able to view the image on a larger screen as well as to take digital images.
• Great For: Seeing details on the surface of objects that are otherwise too big to bring to a standard microscope. For example you can view the hair on someone’s head or details of the skin on their arm.
• Neat to look at: The surface of larger objects that can not be moved to the stage of a conventional microscope. Rocks, Minerals, Insects, Human Beings, Electronics, other small objects.
• Magnification: 5-300X (though usually more like a field scope limited around 100X)
• Best Kind: Units that have a stage as well as a portable capability.
• Worst Kind: Models that aren’t much more than a low quality web camera with snap on optics. Always look for the highest resolution digital camera on board as possible.
• Notes: A USB Digital microscope is probably the best compromise between all features of all the different kinds of microscopes. Since it is connected to a computer you have a large screen to view things on and share with the whole family as well as the ability to take digital pictures and use them in lab reports or science fair experiments. The best kind are those that have high magnification and also have a snap-on stage for use with slides as more of a conventional microscope. Something to consider when looking at digital microscopes is the quality and features of the software that is bundled with the hardware. Look for online reviews for the specific unit you are interested in and let that help you make your decisions.

Optical/Digital Microscopes

• Purpose: The same as a conventional optical microscope only with a digital camera built in.
• Great For: All of the same things as an optical microscope but with the added benefits of a digital microscope.
• Neat to look at: Same as an optical microscope.
• Magnification: Same as an optical microscope.
• Best Kind: A quality optical microscope that has a digital camera replacement for the eye piece. Look for all metal construction, glass optics, LED light source.
• Worst Kind: Plastic construction.
• Notes: An Optical/Digital Microscope is best for a “lab” or “classroom” type environment with the added benefits of a digital camera and computer-screen display. It has the same limitations as a traditional optical microscope but is slightly better for use with slides and traditional biological applications (like viewing cells).

My personal recommendation is to pick between a hand-held digital microscope with good resolution and a high magnification rate and an optical/digital microscope. The key difference is general magnification vs. biological application though the hand-held unit that has a stage attachment can be used for biological applications too. Overall always look for quality construction, glass optics, metal stage, and direct LED lighting. Don’t forget that prepared slides (especially for biological application) can be an important add-on, as well as slide preparing tools. Generally glass slides are better though you won’t want to pick glass slides with younger experimenters. Always teach your children proper care, use, and safety when using any scientific or technical instrument and always supervise younger children.

Identify It! answer for 12-08-2009

Another round of great guesses. The answer to this Identify It! Challenge is:

Identify It! for 12-08-2009

These items are Nuvistors. Nuvistors are the smallest mass produced vacuum tubes and were invented and produced by RCA for use mostly in consumer electronics. Nuvistor is a play on words including “Nu” which sounds like “new” and vistor which sounds like the end of transistor. “Nu” is also similar to “Mu” which is a symbol which stands for the amplification factor of a vacuum tube.

Unlike most vacuum tubes Nuvistors are enclosed inside a metal shell that both provides structure and shields the elements inside the tubes from radio frequency interference. Being metal they are a little more rugged than glass tubes, although unlike transistors they are prone to problems due to vibration. Nuvistors are unique in their manufacture in that instead of each tube being evacuated of air individually and then sealed, entire sets of tubes were placed into a large vacuum chamber, the air removed from the chamber, and then each tube sealed roboticly before all tubes were removed from the chamber. Being small reduced weight and due to the physical smallness of the elements within the tube (cathodes, grids, plates) Nuvisors worked well with high frequency (such as VHF and UHF frequencies used in television transmission.)

Like all tubes, Nuvistors contained a heater, cathode, at least one grid, and a plate. The heater was a wire made of tungsten which is not unlike a filament in a light bulb. Electricity passing through the filament to glow and produce heat. This heat is transferred to the cathode a metal sleave that is wrapped around the filament (but not electrically connected to it). The Edison Effect (noted by Thomas Edison) is that any metal heated in a vacuum will release a charge of electrons into space. When the heater heats the cathode it produces electrons (which can be replenished by supplying the cathode with a fresh supply of electrons by electrically connecting it to a negative voltage supply). As the electrons are emitted from the cathode they can be attracted to another cylindrical conductor known as a plate if it is charged to a positive potential. The plate is connected to a positive voltage source which attracts the electrons from the cathode. Between the cathode and plate are one or more “grids” which are screens of wire. The grid can be connected electrically to an outside voltage source. If the grid is at a potential that is more positive than the cathode but less positive than the plate then electrons can be made to flow to the plate. Since electrons are repelled by negative charges if the grid is connected to a potential that is more negative than the cathode then electrons will be repelled and will not reach the plate. Thus a vacuum tube can act as an amplifier since a small signal applied to the grid can control a larger voltage flowing from cathode to plate.

In the late 1950′s and early 1960′s transistors existed and were being used in electronic devices, however they were not as reliable at high frequencies and were more expensive to produce. Vacuum tubes were still less expensive for a time due to the mass production facilities that already existed for their manufacture.

In our picture we show a 2CW4 and a 6CW4. Both are triodes. The first number indicated the filament voltage, one being 2.1 volts and the other 6 volts. The 4 indicated four elements inside the tube, a filament, cathode, grid, and plate. These tubes are triodes and were used as part of the tuner circuit of early color television sets.

To learn more please visit these pages:

http://www.thevalvepage.com/valvetek/Nuvistor/nuvistor.htm

A reproduction of an article from “Practical Television” magazine from December 1962. This article has a great diagram showing the construction of Nuvistors as well as example circuits that used them.

http://nuvistor.org/1630.htm

An article with some excellent pictures of “Acorn Tubes”… a style of vacuum tube that were the smallest tubes before the production of the Nuvistor. Also some interesting links to some reproductions of 1960′s era electronics magazines relating to amateur radio and “high frequencies”

http://en.wikipedia.org/wiki/Nuvistor

Wikipedia entry on Nuvistors which includes a useful list of part numbers and types.

(insert standard disclaimer on wikipedia article accuracy and non-primary-ness here)

Identify It! Answer for 11-20-2009

Thank you everyone who guessed in this week’s “Identify It!” challenge. Sorry that its taken us longer to get our answer posted than usual but its been a busy week getting ready for Thanksgiving. We’ve done three of these challenges so far and I’m always interested in people’s responses and their thought process.

We had some really good guesses this week but amazingly I was able to stump the fans because no one guessed the correct answer. I was actually expecting this one to be guessed very quickly but I guess we don’t have very many hardcore geologists or mineralogists in the audience at the moment.

Here is the answer: The object in question is a 6 inch diameter by 4 inch tall crystal formation of the mineral Fluorite. This formation is partially transparent (around some corners of some crystals) but is mostly translucent having a purple to violet color. It exhibits a cubic crystal formation typical of fluorite. More images (showing translucency and better coloration) can be found at our fan page on Facebook under the photo’s tab inside the “Rocks and Minerals” folder.

(see: http://www.facebook.com/album.php?aid=125560&id=86177094101 )

Another View of our Fluorite Crystal

Fluorite is a widely occuring mineral that is often found in large deposits. It is a halide made of calcium fluoride (CaF2) and is found most often as cubic crystals though octahedral and more complex forms are possible. Fluorite occurs in a wide variety of colors including colorless, yellow, green, blue, purple and more rarely black, brown, red, pink, and white. Some fluorite samples fluoresce under ultra-violet light (fluorescence is actually named after fluorite, one of the first minerals discovered to posses this trait)

Fluorite uses include: ornamental use, as a gemstone, as a flux in steel and aluminum production, as a substitute for glass in some high quality optics applications, in some semiconductor manufacturing processes involving ultraviolet light, and in the production of hydrofluoric acid.

You can learn more about Fluorite and find pictures of very interesting samples at the following links:

A data table containing lots of information about Fluorite: http://www.minerals.net/mineral/halides/fluorite/fluorite.htm

Pictures of museum samples of Flurite: http://hyperphysics.phy-astr.gsu.edu/HBASE/Minerals/fluorite.html

Bob’s Fluorite Gallery: http://www.peaktopeak.com/fluorite/index.php3

Lots of information and pictures of Fluorite: http://www.galleries.com/minerals/halides/fluorite/fluorite.htm

Wikipedia Entry on Fluorite: http://en.wikipedia.org/wiki/Fluorite

(standard disclaimer: Wikipedia is not a primary source and since the site is publicly editable by anyone the accuracy of information should be called into question. Nevertheless Wikipedia is an interesting experiment as a publicly edited and maintained information repository similar to an encyclopedia and can provide interesting links to other primary sources. Please use appropriate caution when viewing articles on Wikipedia)

Thanks again to everyone who guessed in this week’s “Identify It!” challenge. Keep an eye on the Harris Educational fan page on Facebook for the next “Identify It!” later next week.

Have a wonderful Thanksgiving everyone!

 

Identify It! for 11-20-2009

Here is our latest “Identify It!” picture:

Identify It! 11-20-2009

Visit our fan page at: http://www.facebook.com/HarrisEducational and make your guesses as to what this object is and how it relates to science or technology.  Be sure to include your thought process and as much detail as possible.  Check back in a few days and we’ll post the answer along with some useful links and resources.

Identify It! Answer for 11-10-2009

Identify It! Answer for 11-10-2009

Some very good deductive reasoning from our fans on this Identify It! And one fan got it right. Hopefully no one reading this will ever have a need to see one of these in real life.

The object is a “Finger Ring Cutter” and is used by paramedics to safely cut a soft metal ring off of a finger if the finger has become swollen due to blunt trauma, broken bone, or allergic reaction. The thumb lever holds a guard in place that slips under the ring. This guard keeps the fine tooth carbide steel blade from cutting the finger as its rotated by the handle. If you turn the handle in the opposite direction it unscrews so that the blade may be replaced. The cutter is made of stainless steel so that it may be easily cleaned or sterilized. You can bet that most fire trucks, paramedics, and emergency rooms have one of these stashed away somewhere. A GOOD First Aid kit in any workshop, lab, or serious work area may also have one.

Most rings are made of gold, a gold alloy, or other soft metals which are easily cut by the carbide steel blade of the ring cutter. The cutter is not as effective on wide rings, very thick rings, or rings made out of materials such as tungsten or titanium which are much harder. Tungsten rings are so hard that they usually have a section made of gold so that they may be cut in an emergency.

Remember, always take off all jewelry before operating any kind of power tool. Materials you work with, or the blades, bits, cutters, or moving parts of machinery can snag a ring and in the worst case cause amputation, in the best case ruin a piece of work or break a finger. Always use care when operating power equipment and hopefully you’ll never see one of these ring cutters in real life!

Check back next week for our next Identify It! Challenge.  Thanks to everyone who participated.

Identify It! for 11-10-2009

Here is our second Identify It! Challenge post.

Here is the picture

Identify It! for 11-10-2009

Visit our Fan Page on Facebook, become a fan, and guess what this object is.

http://www.facebook.com/HarrisEducational

Check back here in a day or two to read the answer and learn more about this photograph.

Identify It! Answer for 10-29-2009

Identify It! Image originally posted on 10-29-2009

Answer: Metal Oxide Rectifiers

Specifically Selenium Rectifiers and Copper Oxide Rectifiers

(A formatted/printable version of this article is available at Harris Educational’s Scribd page at:

http://www.scribd.com/doc/21994109/Identify-It-10-29-2009 )

The large square green stack is about 2” on a side and was removed from an unrepairable Motorola 17” TV from the early 1950′s (similar model to 17T3). The light blue stack in the front of the picture is about 1” on a side. It is no longer functional and was removed from a 1950′s Sherwood model 36 high fidelity monaural audio amplifier. (The amplifier was repaired by replacing the selenium rectifier with a pair of high current silicon diodes). The “round” green stack is actually a copper oxide rectifier and was removed from a 1960′s era rotary converter (also sometimes known as a dynamotor, a device for converting low DC voltage to a higher AC voltage and then back into a higher DC voltage). The other rectifiers were removed from other 1950′s and 1960′s era televisions and other equipment.

So What is it?

Metal Oxide Rectifiers are electrical devices that allow electrical current to only flow in one direction in a circuit. They are analogous to one-way valves as used in an air or vacuum pump.

Metal oxide rectifiers are used in power supply circuits to convert alternating current (A.C.) into direct current (D.C.) They were invented in 1933 many years prior to the invention of the semiconductor diode. Selenium rectifiers were used up until about 1975 in place of vacuum tube diodes in electronic devices such as television sets and photocopiers because they were more efficient (about 85% vs. 60% for vacuum tubes), could handle a larger amount of current, and are “instantly-on” since they don’t have to warm up like a vacuum tube does before it can operate. Although they are mostly obsolete today they are still manufactured and used in applications that require rectification of a large amount of electrical current such as in battery chargers and welding equipment.

Most Selenium or Copper Oxide rectifiers were put together as a stack of individual cells. Each cell could withstand about 20 volts. By adding multiple cells in series the voltage rating could be increased almost indefinitely. Selenium rectifiers could handle a larger amount of current (around 50 milliamperes per square centimeter, 50ma/cm2) than Copper Oxide rectifiers and replaced them in most applications. The large metal plates (either square or round) function as a heat sink, providing a large surface area to transfer heat to the surrounding air.

Each cell in a Selenium rectifier is made up of a pressure plate made of either aluminum or steel. This plate is then covered with a very thin coating of another metal such as lead, bismuth, or nickel. A much thicker layer of selenium (usually doped with a halogen) is first heated in an annealing process to form tiny gray hexagonal crystals and is then deposited as the next layer. Another steel or aluminum plate is added next in the stack, as is a steel heat sink plate.

Exploded Diagram of a 2-cell Selenium Rectifier

How does it work?

First some quick definitions:

Voltage is a measure of electrical potential energy (also sometimes called electromotive force or EMF). Since voltage is potential energy it represents the ability to do work with electricity. Voltage is measured in units of “volts” named after the experimenter Alessandro Volta.

Current is a measure of electricity in motion. Current is measured in units called “Amperes” (Amps for short) which is named after the experimenter Andre’-Marie Ampere. Current is a measure of the FLOW of electricity, i.e. how many electrons pass a given point in one second. The movement of electrons through a circuit is what causes useful work.

Electrical Engineers say that when electricity flows from a negative terminal of a power supply through a circuit and then back into a positive terminal the electrical current is called D.C. or Direct Current. D.C. has two characteristics:

1. It flows in one direction only
2. Once a D.C. power supply is turned on, the voltage rapidly rises to 100% and remains there while the circuit is in operation. In other words the voltage does not usually fluctuate.

The electricity we use in our homes is called Alternating Current

A.C. (Generator Position vs. Voltage Output) over 1 rotation

A moving magnetic field causes an electrical current to flow through a conductor. By rotating a coil of wire inside a magnetic field a changing voltage can be generated. When the coil is perpendicular (at right angles) to the magnetic field zero voltage is generated. As the coil rotates into a position parallel to the magnetic field a maximum voltage is generated. As the coil continues to rotate to 180 degrees (again perpendicular to the magnetic field) the voltage drops to zero again. Then as the coil moves to 270 degrees it is again parallel to the magnetic field, but facing in the opposite direction. At this point a maximum voltage is again produced only this time it is negative since it is flowing in the opposite direction. As the coil returns to zero degrees rotation the voltage again drops back to zero and the cycle starts over again. Generators spin fast enough that the electricity we use in our homes is changing like this sixty times in one second.

We use A.C. like this because D.C. is susceptible to losses over distances due to the resistance of the transmission lines. Devices known as transformers can be used with A.C. to step the voltage and current up and down. This makes for a much more efficient system of power transmission.

Using A.C. to transmit electrical power works well BUT many electrical devices and circuits need D.C. in order to operate properly. The process of changing an alternating current into a direct current is called rectification and is accomplished with circuits made up of devices like Selenium Rectifiers, Vacuum Tube Diodes, or Semiconductor Diodes. Since a rectifier only allows an electrical current to flow through them in one direction they may be arranged like one-way valves in order to change the A.C. into a pulsating D.C. The addition of filter capacitors and inductors can smooth out the pulsating D.C. into a usable source of D.C. electricity.

Half and Full Wave Rectifier Circuits

The physical interface between the selenium oxide and the other metal forms a kind of semiconductor junction due to the different electron affinity of the two dissimilar metals. This exchange of electrons across the junction means that electrons can easily be made to flow in one direction but are largely blocked from flowing in the opposite direction. This electron exchange process at the junction is what allows a metal oxide rectifier to function as a rectifier.

Selenium rectifiers were capable of handling a larger amount of current and voltage than early semiconductor diodes due to the large surface area of their junctions and the accompanying heat sink plates that carried destructive heat away from the junction. However over time the junctions in Selenium rectifiers do break down due to chemical changes caused by the remaining heat and the electrons traveling through the junction.

Metal oxide rectifier’s failure rate turned out to be higher than acceptable for consumer electronics and so they were phased out as soon as more reliable semiconductor diodes and rectifiers became available in the mid 1970′s. When a Selenium rectifier fails electricity can flow in both directions across the junction. This means that it acts as a conductor in its normal forward bias direction, but then starts to act as a resistor in its reverse bias direction. The resistance starts off as a very high value but as the junction fails it reduces until it becomes a complete short circuit. Once this happens the metals in the junction are heated to the point that they emit a small cloud of toxic smoke that usually has a tell tale “rotten eggs” smell. Repair technicians could often identify the failure of a Selenium rectifier simply by the smell inside a piece of equipment where one had failed.

Vocabulary Words used in this Article

• Alternating Current (A.C.): Electrical Current produced by a generator that has a voltage that changes magnitude and direction periodically usually as a sine wave function.
• Anneal: A process in metal working in which a desired texture, consistency, or harness is produced by gradually heating and cooling. Similar to tempering.
• Capacitor: An electrical device that stores charge and therefore opposes changes in voltage. Capacitors have two metal plates that are separated by a non-conductive dielectric material. Since capacitors oppose changes in voltage they are used as filters in A.C. to D.C. power supplies to help smooth a pulsating D.C. into a more flat D.C.
• Copper Oxide Rectifier: A largely obsolete electrical device made up of a junction between copper and copper oxide that functions as a kind of surface junction diode to rectify A.C. to D.C.
• Diode: The word “Diode” originally referred to a vacuum tube that had two electrodes, a cathode and anode. These diode tubes functioned as rectifiers because they only allowed electrical current to flow in one direction (from cathode to anode). Today diodes refer generically to any two terminal electrical device that has the property of allowing current to flow in only one direction. Diodes are now mostly made of semi-conductor materials.
• Direct Current (D.C.): Electrical current that flows in only one direction in a circuit. Batteries produce Direct Current.
• Doped: A material is doped if it was originally a pure material but has had an impurity intentionally added for the purpose of creating a semiconductor (effecting how electrons may flow through the material).
• Inductor: An electrical device (usually a coil of wire) that has the effect of opposing changes in electrical current. Inductors are used along with capacitors in power supply circuits to help smooth pulsating D.C. into a more pure D.C.
• Peak to Peak: The total positive difference in magnitude between the highest positive and negative peaks of a periodic function such as a sine wave. One measure of A.C. electrical power is its peak to peak value, though in practice the RMS value is more often used.
• Pulsating D.C.: An electrical current that while flowing in only one direction does vary periodically between a minimum value (usually zero) and a maximum value. Filters made up of capacitors and inductors are used to smooth pulsating D.C.
• Rectifier: An electrical device that allows electrical current to easily flow in one direction but that offers a very high resistance in the opposite direction.
• RMS: Root Mean Square is the mathematical process of taking the square root of the average of squared values of a periodic function such as a sine wave. For a sine wave this is roughly 70.7% of its peak to peak value. RMS Voltage is sometimes called effective voltage because it is the equivalent to the D.C. voltage capable of doing the same electrical work.
• Selenium: Element number 34, with symbol Se and an atomic mass of 78.96. It is related to Sulfur and Tellurium.
• Selenium Rectifier: A now largely obsolete rectifier made of Selenium Oxide that forms a kind of junction diode.

Learn More! (useful links and resources)

• Selenium Rectifier Entry on Wikipedia http://en.wikipedia.org/wiki/Selenium_rectifier
• An article about safety when handling Selenium Rectifiers in functioning electronic equipment originally posted to the newsgroup rec.antiques.radio+phono. http://yarchive.net/electr/selenium_rectifiers.html
• The Road to the Transistor (by Jed Margolin): An article that traces the history and development of semiconductor devices. http://www.jmargolin.com/history/trans.htm
• Cougar Electronics Corporation: One of the few remaining manufacturers of Selenium Rectifiers http://www.cougarelectronics.com/selenium.htm
• Metallic Rectifier Design and Application by Julian Loebenstein.  Published by the Radio Receptor Company, Inc (A former manufacturer of Selenium Rectifiers) Scanned and Formated Document hosted at Scribd http://www.scribd.com/doc/8092683/Selenium-Rectifiers
• Sarkes Tarzian was an American born engineer, inventor, and broadcaster. The company that bore his name, located in Bloomington Indiana was a major supplier of Selenium rectifiers to the consumer electronics industry. http://en.wikipedia.org/wiki/Sarkes_Tarzian
• A Dynamic Periodic Table of the Elements ptable.com http://www.ptable.com
• The Entry for Selenium at periodic table.com http://www.periodictable.com/Elements/034/index.html
• The Entry for Selenium at Wikipedia http://en.wikipedia.org/wiki/Selenium
• An article about Copper Oxide Rectifiers from “Physical Review Online Archive” originally published in 1951 http://prola.aps.org/abstract/RMP/v23/i3/p203_1
• Rectifier and Rectifier Circuits at Wikia Technology/Wikitronics http://electronics.wikia.com/wiki/Rectifier

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