SOMEONE once said that all the equipment a scientist really needs is a pencil, a piece of paper-and a brain. James Clerk Maxwell and Albert Einstein managed to push back the frontiers of knowledge without a cosmotron to their name. For experimental research in nuclear physics today a pencil is not quite enough, but a resourceful amateur can put himself in business with a capital investment of less than a dollar. For this sum he can build a cloud chamber to trap atomic particles. The chamber will stage a show more fascinating than a 100-gallon tank of tropical fish, and in a few sessions will display more nuclear tracks than the observer can analyze in a lifetime.

This simple, recently invented workshop version of a basic physicist's tool is called the diffusion, or "continuously sensitive," cloud chamber. It can be built in 10 minutes by anyone with a dime's worth of dry ice, some alcohol, even the rubbing kind, and a glass jar. Within a few minutes it will begin recording tracks, provided it has been set up properly. The project requires care, close attention to the instructions and some knowledge of cloud-chamber principles. The background and instructions are supplied in the following discussion by I. Clyde Cornog of the University of Pennsylvania's Randal Morgan Laboratory of Physics.

"Toward the end of the last century," says Dr. Cornog, "a few persons here and there were studying the condensation of supersaturated vapors. It was known that water vapor could be cooled well below its dew-point without condensation, provided no dust particles were present. In 1896, C. T. R. Wilson of Cambridge University discovered that the ions formed in gases by X-rays could, like dust particles, act as centers of condensation in supersaturated water vapor. Later he showed that ions produced by 'rays' emitted from radioactive substances would likewise cause condensation. In a supersaturated chamber moving particles produced a visible trail of ionization. The apparatus Wilson developed to carry on this work became known as the Wilson expansion cloud-chamber.

"The phenomenon of radioactivity also was discovered in 1896, and the radiations emitted from radioactive substances were soon identified as of three kinds: negative particles (electrons) positive particles (alpha particles) and gamma rays. Wilson and a colleague, P. M. S. Blackett, found that all three kinds of emission caused ionization, and they saw in the cloud chamber a way of investigating the properties of these emissions-their velocity, energy, charge and so on. They published their first expansion-chamber photographs, showing the white trails produced by particles in 1912.

"The construction and operation of a simple Wilson cloud chamber is shown in the accompanying diagram. The chamber is a glass cylinder with a glass top. Its floor is a movable piston, covered with black velvet as a background against which to see the tracks. The chamber is illuminated by a flat, wide pencil of light. A little water on the velvet serves to keep the water vapor in the chamber saturated (not supersaturated). And a battery is wired to the top and bottom of the chamber; its function is to create an electric field to keep the chamber swept clean of all ions except when it is expanded to show the passage of particles.

"The chamber operates as follows: The piston is pulled down suddenly, increasing the volume of the chamber by about 25 per cent. This cools the vapor and causes it to become supersaturated. Now a particle traveling through the chamber will leave a thin white trail of vapor globules, each representing the condensation of an ion. As a source of particles we can use a speck of some radioactive material which continuously emits beta-particles, or electrons. Close examination of the photograph of the track of such a particle will show that numerous pairs of ions have been formed. When an alpha-particle, which is considerably heavier, passes through the chamber, we see that its track is much thicker than that of an electron, indicating that a great many more ions per centimeter of path have been formed. Gamma rays produce numerous short paths throughout the chamber, indicating that the rays have not only ionized gas molecules but have also given the ions sufficient energy to form tracks of their own.

"If a magnetic field is directed vertically through the chamber, the fast moving charged particles will follow curved paths. By measuring the curvature of these paths it is possible to determine the speed and energy of each particle.

"Wilson cloud chambers have various forms, horizontal or vertical, and range in diameter from 4 to 36 inches. Practically all of them today are automatic, making expansion after expansion (two to four per minute), hour after hour. A photograph of the tracks is usually taken at each expansion. The investigator hunts carefully through thousands of photographs for significant peculiarities in the tracks which may lead to clearer understanding of what has happened.

"As a recorder of events the Wilson expansion chamber is very inefficient. Even taking a photograph every 20 seconds (with an exposure of .001 second), it is operating only one-20,000th of the time. Its efficiency can be improved by having the events one wants to observe trigger the camera. Specially designed Geiger counters, for example, can be coupled to the camera's shutter through a suitable amplifier so that the arrival of particles trips the shutter automatically. The chamber then resets itself in readiness for the next event. But even with this improvement hundreds of events may occur unrecorded between successive expansions.

"Therefore physicists have for some time been working to develop a continuously sensitive cloud chamber. One problem with this modification of Wilson's idea is to devise a way of wiping out old tracks without interfering with the new ones. Great strides have been made recently in the design of sweep fields and other accessories that improve the diffusion chamber's operation.

"In its basic form the continuously sensitive chamber is very simple to make and operate. Almost every laboratory now has one or more. Some operate continuously for the benefit of visitors; some are set up by students; some are used in research projects of the highest importance. Any person can set up such a cloud chamber by carefully following the instructions.

"The basic principles are these: Suppose that a closed vessel is arranged so that the top is warm and the bottom quite cold, and so that a vaporizing liquid is at the top, inside the vessel. This produces a sharp change of temperature inside the vessel, the liquid at the top being near room temperature, and that at the bottom about 70 degrees below zero Centigrade. After a time the vapor formed at the top is found to be continuously diffusing downward, becoming colder and more saturated as it moves. Near the bottom of the vessel it becomes supersaturated, and in that zone the path of an ionizing particle will show up just as it does in the expansion chamber. If these conditions are maintained, and if the sensitive region near the bottom is properly illuminated, radiations entering the vessel will be continuously observable.

"The simplest sort of continuous diffusion chamber consists merely of a large glass beaker, about eight inches in diameter and ten inches high. Inside, on the bottom, is a piece of black velveteen to increase the visibility of the tracks. Across the top of the jar is placed a sheet of cardboard, which is kept saturated with alcohol. The jar is set on a cake of 'dry ice.' Tracks appear in the sensitive region near the bottom a short time after the device is placed in operation.

"The diagram on the opposite page shows a somewhat more elaborate setup. The bottom of the glass cylinder here is an aluminum plate 1/8 inch thick. It is covered with black velveteen inside and fixed to the cylinder with electrician's tape. The top is a similar plate, with a sheet of sponge rubber about the same size as the cylinder fixed to the underside with screws. It is saturated with alcohol. A metal pan of warm water sitting on the top plate keeps the alcohol warm. This apparatus is connected to a battery of 100 to 300 volts which supplies a sweep field. Lights illuminate the sensitive section-a region near the bottom from 1/2 to 2 inches deep.

"To operate the chamber you soak the sponge rubber in alcohol, set the chamber on the cake of dry ice and fill the pan on top with water at room temperature. At this point enters the well-known 'first law of research'-sometimes called 'Murphy's law.' The law may be stated roughly as follows: 'If anything can go wrong, it will.' The alcohol bottle will upset, the chamber will crack, or something else will happen. At any rate, you wait and hope for the best. After a time the temperature gradient is established, the dust particles disappear and you begin to see tracks as you look down into the vessel toward the side where the light originates.

"These tracks are from random nearby radioactivity or from cosmic rays. For a more regular source of radioactivity you may use a little piece of the hand from a luminous watch or clock; the 'Westclox' hands are said to show considerable radioactivity and can be had from watchmaker supply houses. Coat a small piece of hand with a varnish not affected by alcohol and suspend it by a thread in the sensitive region. [SCIENTIFIC AMERICAN will supply a speck of radium, suitably mounted, free-of-charge to any reader upon receipt of a self-addressed, stamped envelope.] The alcohol can be either ethyl or methyl; even denatured (wood) alcohol will serve."

This department has built several successful diffusion chambers based on Dr. Cornog's description, but in every case only after some sharp tussles with Murphy's law. Many things can go wrong- and they do. The interval between completing a chamber and the time it starts to work will be shortened considerably if the builder gives some attention to these details:

1. The vessel should be reasonably airtight. A wide-mouthed jar fitted with a screw top of metal and turned upside down works better than one which merely sits on a metal plate. If the latter arrangement is used, wet the metal plate with alcohol to form a liquid seal.

2. The importance of side-lighting cannot be overemphasized. The beam from a home movie or slide projector makes an ideal lighting arrangement. The important point is to light the individual droplets brightly in comparison with their surroundings.

3. The experimenter must learn to recognize the trails of droplets as they appear. Soon after the apparatus is set up, clouds will form two or three inches above the bottom of the vessel and "rain" will begin to fall from them. This micrometeorological phenomenon will be enhanced if the velveteen carpeting on the floor of the chamber is moistened with alcohol. The rain is easy to recognize and the tracks appear as thin, silvery threads in its midst. They last for only a split second, then drift away like miniature wisps of smoke.

4. If you use an up-ended jar as the chamber, it is advisable to cement the alcohol pad to the inner surface at the top and the velveteen covering to the inner surface of the metal lid. The cement must be allowed to dry thoroughly, else it may contaminate the alcohol and thus ruin the experiment. Another, and better, way to hold the parts together is to use a wire expansion ring or some similar device.

5. Remember that there must be R sharp difference of temperature between the top and bottom of the chamber. The assembly should rest directly on the block of dry ice. A metal bottom, because of its superior heat conductivity works better than one of glass or plastic. The dry ice should stand on some insulating material such as corrugated cardboard, and the uncovered portion of the top of the ice should be covered with a bit of cloth to prevent frozen vapor rising from it from interfering with observation.

6. A well-soaked pad holds enough alcohol to keep the chamber going for about eight hours. As previously mentioned, the metal plate at the bottom should be moistened, but not flooded with alcohol, as a thick layer of alcohol tends to reduce the depth of the chamber's sensitive region.

7. It is easy to paralyze the chamber with overly "hot" radioactive material. The amount of luminescent material in the minute hand of a Westclox can stop the chamber's activity at a distance of 18 inches! Use only a tiny chip of it. However, in most localities tracks induced by cosmic rays will provide enough excitement to reward the amateur's effort in constructing the device.

IN 1946 Lieutenant Commander Eugenio Conceiçao Silva, of the Bairro dos Oficiais, Alfeite, Portugal, wrote to this department that, after building several reflecting telescopes from 4 to 12 inches in diameter with the help of Amateur Telescope Making, he wanted to "try his hand and elbow grease" with a 20-inch and needed a glass blank from which to make the concave mirror. Since then we have exchanged numerous letters, and the following account brings Commander Silva's project up to date.

It took him 18 months to obtain a 20 inch blank; he finally purchased one for £36 (about $90) from Chance Brothers, Limited, of Birmingham, England. While awaiting its arrival and, as he writes, "working backward in the good old fashion set up by ATM," he first prepared on the roof of his house an 18-foot, metal-roofed, wood-lined dome turning easily by hand on 13 ball bearings.

Four years later Commander Silva wrote that he had finished his telescope and had it in working order, "ready for star-gazing, variable-star-observing and photography." He sent a collection of photographs made at its Newtonian focus. Three of these are reproduced on pages 184 and 186. That the telescope gives a high-grade optical performance is shown by the small round star images in the corners of the photographs. (Some of the images lose their roundness in the halftone engraving process.)

"I chose the double-yoke English type of mounting," Commander Silva writes in his perfect English, "because of its sturdiness and ease of construction and despite its one limitation, the inaccessibility of the Pole Yet it can be pointed as far as 75 degrees north declination." The yoke of the telescope is built of half-inch oak plywood. The octagonal tube is built of the same material reinforced by iron ribs. The focal length of the mirror being 10 feet, the focal ratio is f/6 A coudé Cassegrainian secondary mirror, when used, converts this to f/18 for visual observing with powers up to 800. There are identical screw-capped openings on four sides of the tube for insertion of the eyepiece-diagonal unit, and four in addition for the coudé unit.

Commander Silva continues: "The mirror blank, originally 3 by 2-1/2 inches, was worked throughout face up with a 12-inch subdiameter glass tool on which I had previously made two 12-inch mirrors, and thus it was worn down finally to only one-half-inch thick. The strokes used were those described in Amateur Telescope Making-Advanced. I found the instructions adequate when supplemented by thought and the usual willingness to experiment, fail, try again and maybe fail again but keep on learning until results are attained. The subdiameter tool method permits a larger element of personal idiosyncrasy than the conventional method. The job was easier than I had expected. The most tedious part was the roughing out, but the grinding went very well with the tool on top. Astigmatism was avoided by walking as regularly or, rather, as irregularly around the mirror as possible. It was always supported on the nine-point equalizing system later installed in the telescope. Polishing and zonal correcting were done with honey-comb foundation, but final figuring was done on a pitch lap since HCF gives bad contact. By the subdiameter lap method the work advances very rapidly, and frequent testing is necessary. Raised zones were treated by pressure on the edge of the lap a little outside of their crest. I used cerium oxide because I was unable to get scratch-free rouge.

"Parabolization with the subdiameter lap proved easier than with full-size laps. By the Foucault test, the mirror was found to be a little undercorrected. I let it stay and called it a job. Having caught chronic 'mirroritis,' I should enjoy making a SO-inch mirror by the subdiameter tool method.

"I am satisfied with the telescope's performance. Visually it easily reaches its theoretical magnitude, 15.5.

"The drive is powered by a fan motor and drives the sector attached to the polar axis through a train of reducing gears and a nut and a screw which may be run back with a hand crank. Here at Alfeite the electric power is not synchronized and the voltage, nominally 220, often changes more than 10 per cent; hence I built an automatic centrifugal regulator or governor to control the drive. The motor is belted directly to the regulator. The belt is not tight and may slip if the motor turns too fast. I had to calculate the position of the points of suspension of the two rotating masses so that for three revolutions per second they would be in equilibrium in any position, but if they turned faster they would suddenly rise and bring the rotating disk against a fixed disk at the top. To obtain this result accurately they should theoretically rise in a parabola instead of a circle. This being difficult to arrange, the points of suspension of each mass must be made to coincide with the center of a circle tangent to the middle of the parabola.

"It was a pleasure, and perhaps a surprise, after the regulator was built, to test it and, counting the revolutions made in one minute, find the designed 180, for I must confess that I was a little doubtful about the results of my geometrical lucubrations. It is, however, possible to change the critical velocity a little if necessary by screwing the two masses up or down on their rods. This is the Young isochronous governor, not the Silva, and it works very well.

"The flow of current may be controlled by two resistances, which may be either taken out or put in the circuit by two buttons held in the hand during photographic exposures. In this way it is possible to drive the motor faster or slower than its normal speed and, notwithstanding the governor, the driving mechanism obeys."

Commander Silva, whose chapter on a double-star micrometer is included in Amateur Telescope Making-Advanced, is a professor of ballistics in the Portuguese Naval Academy and director of the Naval Laboratory of Explosives. He mentions that his hobby resembles his vocation-both consist of aiming hollow cylinders at the sky. "A few months ago," he says, "I wrote some articles on telescope-making in a Portuguese popular science magazine. As a result there are now seven fellows grinding glass disks and hoping to see the moon at arm's length."

Instructions for calculating the design of the isochronous governor, with a worked-out example, which Commander Silva kindly supplied by request, are held over for later publication for lack of space, as are descriptions of his solar eyepiece, his method of making Ramsden eyepieces, and his solar prominence spectroscope with a Thollon (carbon disulfide) prism.

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