High-voltage Spark Generator (Dec, 1930)
High-voltage Spark Generator
By WalterE. Burton
Here’s a simplified Toepler-Holtz static machine for generating high-voltage electric sparks. By experimenting with it you can learn something about the nature of electrical currents.
IF YOU are interested in learning the nature of that mysterious and invisible force known as electricity, there is no better means of studying it than by experimenting with this inexpensive and easily made static machine. This is not a machine, mind you, which makes that awful noise in your radio receiver, but a device which generates high voltage electric sparks which are quite harmless, but with which you can have barrels of fun. It is of the Toepler-Holtz type which is used in most school physics labs to demonstrate the strange things that electricity can be made to do.
Where the Toepler-Holtz machine uses glass discs, this generator uses old twelve-inch phonograph records that are too worn and old to be of any further use. After the surfaces have been scrubbed thoroughly with soap and water and a stiff-bristled brush to remove dirt and bits of metal that might be hiding in the grooves, several coats of shellac are applied to one side of each disc, each coat being allowed to dry thoroughly before the next is applied.
While the shellac is drying you can con-struct the woodwork that holds the rear disc rigidly in a vertical position while the front one is being revolved rapidly to generate electricity. The bottom portion of the baseâ€”the base on which the entire machine is mountedâ€”is simply a board 7-1/2 inches wide and 16 inches long, with edges slightly bevelled. Halfway between the ends, and at one side of the center, an upright, 2×2 inches at the bottom and nine inches high, is mounted, as shown in Fig. 1. This upright supports the stationary record and the belt pulley driving mechanism for the rotating one.
Through this wooden upright, two 1/2-inch holes are bored, one near the top and the other near the bottom. Into the lower one a 3-1/2-inch section of 1/2-inch shafting is wedged, so that a large diameter pulley wheel can turn on the 3/4-inch portion that projects beyond the surface. A similar shaft about four inches long is inserted in the upper hole, on one end of which is mounted the revolving disc, as shown in Fig. 5. A small pulley is mounted on the outer end of this shaft as shown in Fig. 1. These pulleys, the shafting, and belt can be purchased at any five cent-to-a-dollar store, or at any mail order house.
With the records dried out thoroughly, and the pulley system completed, you are now ready to go to work on the records again. First cut ten discs from tinfoil, each 2-1/2in. in diameter, and two strips, each 3/4 in. wide and about four inches long. Using shellac as an adhesive, mount six of these discs on the shellacked face on one record, spacing them evenlyâ€”60 degrees apartâ€” with the edges of the tinfoil about 3/4-inch from the outer edge of the disc, as shown in Fig. 2. These form the plates on the rotating disc.
To the shellacked surface of the second record, mount four of the discs and the two strips, so that the arrangement resembles two dumb-bells parallel to each other, and opposite sides of the center hole as illustrated in Fig. 1.
Next ream out the center holes of the records to a diameter of 1/2 inch, then drill two smaller holes on each side of the larger ones, about 3/4-inch from the center. Through the center holes in the disc with the dumbbell shaped armature plates are inserted flat headed bolts by means of which the stationary disc is fastened to the rear wooden upright. The corresponding holes in the disc with the six tinfoil discsâ€”the one which revolvesâ€” takes the bolts which fasten it to the wooden disc, as shown in Figs. 2 and 6. This wooden disc is attached to the shafting by means of a pin.
With this arrangement completed you should be able to rotate the disc with the six tinfoil plates by turning the crank on the rear wooden upright.
The next job is to make the four brushes which produce the static charge by rubbing against the outer disc as it revolves. These consist of lengths of fairly stiff copper wire bearing little bundles of tinsel, such as that used in trimming Christmas trees. Two of these brushes are attached to the ends of the small brass strip which extends around the edges of the discs and are bolted to the center of the tinfoil dumb-bell on the rear record, as shown in Figs. 1 and 6, which give rear and front connections, respectively.
The two other brushes are fastened to the ends of the neutralizing rod, as shown in Fig. 3. This neutralizing rod is a brass or copper strip, seven inches long, and fastened to the front upright in such a position that the two tinsel brushes at the ends form an electrical connection between the two opposite tinfoil discs.
The task of making the collectors for taking off the electric charge and storing it in the Leyden jars now presents itself. These consist of short pieces of copper screen wire, as shown in Figs. 3 and 6, that are supported so that one is on each side of the center, opposite the foil discs, and at the same height from the base as the center of the record. The screen wire points are placed so that they are close to the record, but do not touch it.
The rods holding the copper screen collectors are supported by a bakelite strip which is screwed to a wooden upright mounted on the front of the base. This same bakelite strip also supports a spark gap made of two long brass rods, on the ends of which are soldered large ball bearings, as shown in Figs. 2 and 3. Also, at the points where the screen collector connects with the spark gap, brass or copper strips are fastened which run to the Leyden jars that serve to store up the electric charge.
The Leyden jars are made by cementing with shellac tinfoil to the inside and outside surfaces of two olive bottles, the foil covering the lower two-thirds of the jars. The two outside foil surfacesâ€”really the plates of the two Leyden jar condensersâ€”were connected together by means of a copper strip, as shown in Figs. 2 and 3. The inside coatings are connected to the collectors by metal chains, shown in Fig. 4.