IF YOU FASTEN A DISK OF alcohol-soaked blotting paper in the bottom of a jar, screw on the lid and up-end the jar on a cake of dry ice, you will occasionally see the concluding phase of an event which may have had its origin millions of years ago in an exploding star. The apparatus comprises a cloud chamber of the diffusion type. Every few seconds a sharp, momentary vapor trail will appear near the bottom of the jar. Most of the trails represent particles from radioactive material in the earth, or fragments of atoms smashed in the atmosphere by cosmic rays. But once in a great while the trail will mark the passage through the jar of a primary cosmic particle from space.

Each of the known nuclear particles leaves its characteristic trail in the cloud chamber, and photographs of them are available in reference texts. A number were illustrated by diagrams in "The Ultimate Particles," by George W. Gray, in SCIENTIFIC AMERICAN of June, 1948. With the help of published photographs of tracks you can identify the particles as you would birds or butterflies. You may find the behavior of the particles just as fascinating.

The vapor trails of particles are like the swath that a cannon ball might cut through a young forest. If you were to view a series of such events from an airplane, you could judge the relative size, speed and energy of the balls by the length and width of the swaths. From the zigzag paths of ricocheting missiles you might learn something about the structure of the forest. Occasionally a swath unlike any previously recorded might signal a new type of cannon ball or gun. That is how the list of nuclear particles has been compiled and how the cloud chamber tips off physicists to the presence of unexplained forces in the universe.

Because dry ice is often difficult to procure, a number of our correspondents have asked for the design of an iceless chamber. A simple one can be built on principles used by C. T. R. Wilson, the inventor of the cloud chamber, in his first version. While at the Ben Nevis Observatory in Scotland in 1894, Wilson became fascinated by the play of sunlight on clouds surrounding the mountain, particularly by circular rainbows around the sun. On returning to the University of Cambridge, where he was senior demonstrator in the Cavendish Laboratory, Wilson attempted to imitate the phenomenon in miniature. His apparatus consisted of three bottles, some tubing and an air pump. He put some water in a large bottle and inside this up-ended a small bottle, also partly filled with water. It was held in place about half way down in the larger bottle. Now the large bottle was stoppered and connected by tubing to an evacuated third bottle. When a petcock was opened, air rushed out of the big bottle into the vacuum chamber. As a result the moist air in the small inverted bottle expanded suddenly, and the air was suddenly cooled. At the reduced temperature, the air contained an excessive amount of water vapor and was in a state of super-saturation. If dust particles or other nuclei were present, the excess vapor would condense on them and form a cloud.

Out of two 12-ounce peanut-butter jars, a coffee can, some tubing and a toy balloon you can make a version of Wilson's apparatus in an hour or less [see Figure 1]. Cut the jars' tops so as to leave the screw-rims, then butt and solder the two rims together with a disk of fly screening sandwiched between. Next drill a hole large enough to take a No. 6 machine screw, through the bottom of one of the jars, and drill a larger hole, about 3/8 inch in diameter, in the side of the same jar. Drilling glass is not difficult. Cut several notches in the end of a piece of brass tubing, chuck the piece in an electric drill and rotate the notched end against the glass while applying a slurry of No. 120 Carborundum grains in water. Avoid wobbling the drill and go easy on the pressure when the drill cuts through the inner wall.

This first jar will be the expansion chamber. In the wall of the second jar cut a hole 9/16 inch in diameter, and fit it with a short pipe nipple screwed to a "street" elbow-a fitting with a male thread on one end and a female other. This piping and the fittings should be of the size known in the trade as 1/4 inch: its outside diameter is 35/64 of an inch and the inside diameter, 3/8 inch. Fasten the nipple in place with a mixture of litharge and glycerin or any of the commercially available rubberized cements. When the cement has set, fit the balloon over the end of the street elbow and tie it in place as shown in the diagram.

A circle of 14-gauge bare copper wire is then fastened to the inside bottom of the expansion chamber and secured by a machine screw. The remaining hole in the chamber is fitted with a stopper. Now make a solution consisting of equal parts by volume of water and alcohol with ink added (to color the fluid black) and a half teaspoon of salt. Fill the second jar with the fluid and partly fill the expansion chamber. Then screw both jars into the cap assembly. The filling job is simplified if you make up a large crock of solution and assemble the apparatus while it is partly immersed in the fluid.

Connect the outer end of the pipe nipple with a source of compressed air. The source is a pumped-up automobile inner tube, but you can make a convenient pressure tank of a coffee can of the one-pound size. The valve assembly old inner tube will serve as an inlet. Solder a pipe nipple into the tank for an outlet and fit it with a shut-off cock ("A" in the drawing) for admitting air to the expansion chamber. A drain cock ("B") is also provided for bleeding the chamber during the expansion stroke.

In preparing the apparatus for operation, first close both cocks and pump the tank to a pressure of about 10 or 12 pounds. Opening the shut-off cock "A" admits air to the balloon and forces the fluid from the lower jar through the screen (which minimizes turbulence) into chamber above. This compresses the air trapped above the fluid, and its temperature increases The warmed air quickly takes up additional moisture from the fluid. Close the shut-off cock and open the bleeder. This relieves the pressure inside the chamber, and air escapes from the balloon through the bleeder valve. Both the fluid and the temperature drop abruptly, creating a state of supersaturation in the expansion chamber. The action differs from that of Wilson's chamber in that the expansion stroke ends with air at substantially atmospheric pressure, whereas expansion in the Wilson chamber is completed at lowered pressure. The higher working pressure gives the "peanut butter" chamber an advantage because the increased density of the gas betters the chance that a nuclear particle will collide with the nucleus of an atom and thereby produce an interesting event.

Beginners sometimes attempt to simplify Wilson's apparatus by exhausting air directly from the chamber, thus avoiding the complication of a piston, either liquid or mechanical. Such schemes invariably fail because the turbulence created by the escaping air destroys the tracks. Piston devices confine the motion of the air to the vertical direction. Small eddies are created near the lower walls of the chamber, but they are not serious.

Wilson took special pains to avoid turbulence when constructing his second instrument. This one had a more accessible chamber, consisting of a glass cylinder and piston. The moving parts were fitted with almost optical precision to prevent air from leaking past the piston into the chamber. Lord Rutherford recalled in later years how, during the early stage of construction, he observed Wilson in his laboratory painstakingly grinding the piston into the cylinder. Rutherford, called away from Cambridge, returned some months later to find Wilson still sitting in precisely the same place, patiently grinding away!

Wilson's third and final design featured a clever solution of the problem of access to the interior of the chamber. Essentially this apparatus consists of a cylinder equipped with a glass top and a close-fitting, free-floating piston. The assembly stands in a shallow pan of water which acts as a seal. The expansion stroke is made by exhausting air beneath the piston through a vent in the center of the pan extending slightly above the level of the water. The length of the expansion stroke is limited by a rubber stop beneath the piston. To reach the interior of the chamber, you simply lift the assembly from the pan and pull out the piston.

This is the chamber Wilson described in the celebrated paper he presented before the Royal Society in 1912. A tribute to the excellence of the design is the fact that during the remainder of Wilson's long career all his nuclear researches were made with this chamber. It is now preserved in the Cavendish Museum. A few years ago Sir Lawrence Bragg asked Wilson whether the apparatus properly be labeled "the original." "I never used or made but one," Wilson replied.

Certain requirements must be observed when operating an expansion chamber. The volume and speed of expansion must be maintained, and stray ions must be swept from the chamber. The expansion stroke of the apparatus diagrammed here is completed in about a twentieth of a second, well within the limit required. It can be speeded by increasing the diameter of the tubing.

Early in his experiments Wilson learned that the supersaturation required for showing tracks of negatively charged particles is reached when the ratio of the chamber's volume before expansion to its volume after expansion is 1.25. At this ratio a few drops form in a dust-free atmosphere. Positive ions will not act as condensation nuclei for water vapor until the expansion ratio reaches a value of 1.31. This requirement can be lowered somewhat by adding alcohol to the water. When the expansion ratio exceeds 1.38, a dense cloudy condensation is produced in the chamber even when no nuclei are present. Wilson usually adjusted his apparatus for a ratio of between 1.33 and 1.36. The expansion ratio of the peanut-butter chamber is established by the amount of air admitted to the balloon and, hence, by the level reached by the fluid in the expansion chamber during the compression stroke. This level is measured by the paper scale cemented to the upper jar.

The chamber is cleared of spurious ions (and unwanted clouds) by connecting a voltage between the copper electrode in the expansion chamber and the salt solution. It is good practice to sweep the field after each expansion and to maintain the voltage across the chamber until the moment of expansion.

The peanut-butter chamber is suggested as a starting point for amateurs who find dry ice in short supply. Its disadvantage, compared to the diffusion chamber, is that its surveillance of particles is not continuous.

Beginners will discover that the tracks show up best when viewed obliquely in an intense beam of collimated light. The irregular end of a peanut-butter thus far won no laurels in the optical goods industry, and the walls are not much better. Professional chambers are equipped with windows of plate glass. If you want to make good photographs of nuclear tracks, you must of course synchronize the camera shutter with the completion of the expansion stroke. Wilson's exposures were made by discharging two gallon-sized Leyden jars through a mercury spark gap. The exposure time was about a thousandth of a second. If you own one of the electronic-flash outfits now popular with photographers, you will be spared the labor of duplicating Wilson's gap.

A self-addressed, stamped envelope sent to this department will bring you a speck of radium suitably mounted for actuating cloud chambers, Geiger counters and other devices.

"For some time," he writes, "I have been experimenting with liquid prisms and have uncovered some information which may be helpful to those who find themselves up against the characteristic difficulties of these devices. The main problem was finding an effective cement to hold the glasses together. It has to be some stuff which sets by cooling, does not require a volatile solvent and is quite insoluble in the liquid with which the prism is filled. For prisms filled with carbon disulfide or bromonaphthalene I have, after many trials, found a very good cement-shellac.

"In making the prisms I first heat the pieces of glass on an electric hot plate. The shellac is then applied to the edges to be joined and, as it begins to melt, is spread evenly with a needle. The hot plate must be regulated for the melting temperature of shellac and controlled so the shellac is neither burned nor comes to a boil. The work must then be cooled slowly to avoid cracking the glass. The joint is very strong, and after cooling the glass will break before the cement if you try to pull the elements apart.

"The two crown-glass prisms of the Thollon design were cut from a glass slab about 30 millimeters thick and hand ground [see lower drawings in the illustration on the left]. The inside faces were fine ground and polished. Then the two prisms were cemented together with the plate glass top cover. The side faces (held together at the top) were then fine ground and the side plates cemented to the assembly. The rim around the base was fine ground and the base cover cemented in place. Finally outside faces were fine ground and polished in a plaster cradle. (These operations are much easier to perform than to describe.)

"The faces of the assembled prism were cleaned with a cotton swab dipped in alcohol, a good solvent for shellac.

"All liquid prisms must be fitted with a small expansion chamber to relieve the inner pressure when the fluid expands and contracts with changes in temperature. My prisms are equipped with the small metal chamber shown in cross-section. The chamber is left empty when the prism is filled. I did not provide for expansion in my first Thollon prism, which was filled with carbon disulfide. When I picked it up, the slight heat from my hand expanded the fluid and cracked the side plates! My prisms measure 30 millimeters in width and 40 millimeters in height. The glass covers, measure two millimeters in thickness.

"As a stopper for the filler hole I use a small sheet of glass. The prisms cannot be heated on the hot plate after filling, so the glass cover is placed over the hole and cemented to the expansion chamber by means of a soldering iron.

"My prisms are now more than three years old and they have developed no leaks. It seems probable that they are as permanent as solid glass ones."

George W. Ginn of Hilo, Hawaii, submits some stereo photographs of the moon. Two sets of his pictures are shown on this page. One is a pair of views which gives the illusion of three dimensions when viewed with the aid of a thin mirror. Stand the mirror vertically with respect to the plane of the page and align the edge with the boundary separating the right and left views. If the glass is a foot high, let the tip of your nose touch the upper edge and look toward the view behind the mirror.

The lower picture is printed for viewing either "cross-eyed" or, in case you are not blessed with this talent, by means of a conventional stereoscope. With a bit of practice, most persons can perfect the art of cross-eyed viewing. Hold the page in your left hand about a foot from your eyes. Now place the tip of your right index finger between the views, focus both eyes on the fingertip and slowly move the finger toward you. When your finger reaches a certain distance, you will become conscious of four indistinct moon images in the background. Move your finger until the inner images merge. Then shift your focus to this center picture. The moon will appear clear, sharp and in three dimensions.

In commenting on his lunar stereos, Ginn writes: "The views were taken with a three-and-a-half-inch objective of 42 inches focal length. My telescope has provision for placing an Exakta camera at the prime focus in place of the eyepiece. The pentaprism finder of the camera makes a fair eyepiece. The image of the moon on the film is three eighths of an inch in diameter and has been enlarged about six diameters.

"The two halves were taken seven hours apart, allowing the earth's rotation to provide the base line. For true perspective they would have to be viewed at a distance of about 20 feet."

Dr. John H. Schaefer, a Los Angeles physician, writes us the following letter:

"I suspect that whoever prepared the captions for the illustration on construction details of an acoustic lens of the gas type on page 122 of SCIENTIFIC AMERICAN for January had his mind elsewhere. At the bottom of the cut the gas generator is shown with these words-'dry ice' for CO₂, marble chips and HCl for hydrogen.' When I went to school, marble chips (calcium carbonate) and hydrochloric acid reacted to liberate carbon dioxide. Zinc and hydrochloric acid liberate hydrogen."

Dr. Schaefer's observation confirms this department's sad experience that editors as well as laboratory workers are subject to Murphy's Laws, to wit:

I. If something can go wrong, it will.

II. When left to themselves, things always go from bad to worse.

III. Nature always sides with the hidden flaw.

Bibliography

AMATEUR TELESCOPE MAKING. Edited by Albert O. Ingalls. Scientific American, Inc., 1952.

ON AN EXPANSION APPARATUS FOB MAKING VISIBLE THE TRACKS or IONISING PARTICLES IN GASES AND, SOME RESULTS OBTAINED BY ITS USE.; C. T. R. Wilson in Proceedings of the Royal Society of London, Series A, Vol. 87, No. 595, pages 277-292, September I9, 1912.

EXPERIMENTAL SPECTROSCOPY. Ralph A. Sawyer. Prentice-Hall, Inc., I944.

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800

Internet: http://www.sas.org/