As I was looking for my first assembly draft for AWAN, I came across an article posted by Noah's Neurosphere, which is basically an excerpt from an issue of Scientific American written by retiring columnist Clair L. Stong. The column article itself was later selected in his book "The Scientific American Book of Projects for the Amateur Scientist" published in 1960.
The original article in Scientific American contains two parts and was titled: A Computer to Solve a Problem of Mechanical Translation and an Ingenious Cloud Chambers. Here, I'm going to extract the part on cloud chamber, verbatim.
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Among readers who requested the samples of radium offered last April in connection with the article on cloud chambers was Louie R. Hull, a physics teacher at South Side High School in Fort Wayne, Ind. He sends some photographs of alpha and beta tracks recorded in his homemade cloud chamber, one of which shows how a barrier of cellophane blocks the alpha particles.
"These tracks," says Hull, "were photographed with the aid of a 'plumber's friend' cloud chamber" - an arrangement assembled from odd parts from the junk box. Like the 'peanut-butter jar' chamber that you described, the plumber's friend can be constructed in a single evening. The basic idea stemmed from a chamber of the rubber-bulb compression type popular for classroom demonstrations. Although the rubber-bulb instrument is satisfactory for a visual demonstration, trouble was encountered when we attempted to photograph tracks with it. Compressions could not be reproduced uniformly, nor could we time the exposures correctly. Various alternate arrangements were tried until it occurred to us that the plumber's friend- a rubber plunger of the kind used for clearing drains-might work. In the cloud chamber it does not operate strictly as a piston does, of course, because the center of the plunger moves a greater distance than the edges. As a result the particle tracks are distorted somewhat; nevertheless we are delighted with the performance of the arrangement and have made hundreds of excellent pictures.
The side walls of the chamber are cut from a quart glass jar by the hotwire method. You wrap a single turn of iron wire (such as that used for binding brooms) around the jar tightly at the place where the cut is desired, the ends being separated by a small sheet of asbestos insulation where they would otherwise make electrical contact. The loop becomes red hot when you connect it across the six-volt terminals of a transformer or storage battery. After about 30 seconds of heating you remove the wire. Plunge the jar into cold water immediately. The glass will break cleanly at the line where it was heated by the wire. The sharp edges are then rounded with abrasive such as emery or carborundum.
The top of the chamber is closed by a plate-glass window [figure above]. If you do not own a circular glass cutter, one can easily be rigged from a wheel-type cutter available in hardware stores. Fasten the wheel end of the cutter to one end of a short length of inch-square wood so that the wheel protrudes slightly beneath the lower edge of the wood.
Next drive a wood screw through the wood vertically at a distance from the cutter wheel equal to the radius of the desired glass disk. The protruding tip of the wood screw serves as the center point of a compass, the wheel as the other point.
Then make an indentation with a center punch in a small scrap of 16-gauge sheet metal. You place this punched piece of metal on the glass to be cut, backing the metal with friction tape to prevent it from slipping. The punched indentation is centered with respect to the glass. Now you put the protruding tip of the wood screw in the indentation and make a circular cut in the glass with a single, firm rotary stroke of the tool. After this, if you make 10 straight radial cuts from this disk to the edge of the glass sheet, you can break away the outer pieces, leaving a disk the same size as the circular cut. You then smooth the edge of the disk with abrasive.
The glass cylinder, window and plumber's friend are fastened together with metal rings [left figure above]. The ring fittings can be built of thin sections cut from sheet metal and soldered. If a metal cutting lathe is available, you can machine them from a thick slab of stock. Rubber gaskets must be inserted where the metal and glass come into contact. Turbulence in the chamber is minimized by inserting a disk of black velveteen, supported by wire screening, between the cavity of the rubber plunger and the chamber. The radioactive sample is supported inside the chamber by a machine screw inserted through the side wall.
The assembled chamber is supported on a wooden bracket. Its expansion is actuated by a lever mechanism, which is tripped by a motor-driven cam. The chamber is illuminated by a 300-watt slide projector, the beam of which is controlled by a shutter released electromagnetically. The camera is positioned above.
To make the compression stroke you lift the horizontal lever quickly and hook it to the vertical lever. The compression should not exceed about one third of an atmosphere, or the chamber will fill with fog on expansion. After about 30 seconds you start the motor. The cam advances until the metal arm at the top of the vertical lever drops into a notch on the cam. A spring then pulls the vertical lever away from the chamber, unhooking the horizontal lever. The plumber's friend then springs to its original shape, accomplishing the expansion stroke.
An electrostatic 'clearing' field is applied to the chamber automatically during the compression stroke by a microswitch actuated by the horizontal lever. The field is removed by the switch and the leads to the chamber are short-circuited automatically at the end of the expansion stroke. Similarly, the motor-driven cam is equipped with switches for operating the projector and shutter release in sequence. Exposure time is fixed by the tension of a rubber band hooked to the shutter.
The proportion and amount of liquid in the chamber are not critical. Good tracks form with either 180-proof grain alcohol or rubbing alcohol as it comes from the bottle. Performance is influenced by room temperature, however. Above 70 degrees Fahrenheit results are improved by diluting the alcohol slightly with water-say 15 drops of alcohol to two drops of water. It should be kept in mind that fog results from too much liquid as well as from over-expansion. The chamber rarely requires more than 20 drops of liquid. Don't expect to see tracks during the first few expansions. The liquid must have time to evaporate.
Beta tracks, being thin, are more difficult to see than alphas and appear best when the alphas have faded. It is interesting to investigate the penetrating power of particles through thin sheets of various materials. The chamber also enables you to experiment with various other atomic phenomena. If you substitute a freshly polished needle of zinc for the radium source, for example, you will see beta tracks shoot from the point These are photoelectrons released by light shining on the metal. The number of photoelectrons ejected will increase immensely if the chamber is illuminate by an arc lamp shining through a window of sheet quartz or of a clear plastic that transmits ultraviolet light readily. Such modifications of the chamber enable you to investigate the photoelectric properties of many substances a well as other forces that disturb the atomic structure of matter.
CROSS REFERENCE
C. L. Stong, "The Scientific American Book of Projects for the Amateur Scientist", Simon and Schuster, 1960, p.p. 314
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