Chapter XIII

FRICTIONAL ELECTRICITY

Frictional electricity is high potential, current alternating, and of high voltage but very low amperage. Apart from certain uses in laboratory and medical practice, it is valueless. In its greater volume it is akin to the lightning-bolt and is dangerous; but in its smaller volume it is a comparatively harmless toy. From the glass rod, or the amber, rubbed on a catskin to the modern static machines is a long jump, and the period of exploitation covers centuries of interesting experiments, most of which, however, have been practically useless for any commercial purpose.

Static or frictional electricity is generated by friction only, without the aid of magnets, coils of wire, or armatures rotating at high speed. The simple process of the glass and catskin has been variously modified, until at last Wimshurst invented and perfected what is known as the “Wimshurst Influence Machine.” It is self-charging, and does not require “starting.” It will work all the year round in any climate and temperature, and is the greatest improvement ever made in static electric machines.

Apart from its efficiency under all conditions, it is the simplest of all machines to make, and can easily be constructed by a boy who is handy with tools, and who can obtain the glass and brass parts necessary in its construction. The principal parts of an influence machine are the glass disks, wooden bosses, driving pulleys and crank, glass standards, brass arms with the spark-balls at the ends, and the base with the uprights on which these parts are built up and held in position.

A Wimshurst Influence Machine

Obtain a stiff piece of brown paper twenty inches square, and with a compass describe a circle twenty inches in diameter. Inside of this circle make another one fourteen inches in diameter, and near the centre a third circle six inches in diameter. Another circle four inches in diameter should be drawn inside of the six-inch circle, so that when the bosses are made fast to the glass plates they can be properly centred. Also mark sixteen lines radiating from the centre, equal distances apart, as shown in Fig. 1.

From a dealer in glass purchase two clear, white panes of glass eighteen inches square. Be careful not to get the green glass, as this is not nearly so good as the white for static machine construction. If it is possible to get crystal plate so much the better. The panes should be thin, or about one-sixteenth of an inch in thickness, and free from bubbles, wavy places, scratches, or other blemishes.

From these panes cut two disks sixteen inches in diameter with a rotary cutter, as described in the chapter on Miscellaneous Apparatus, page 294, and rub the edges with a water-stone (see chapter on Formulæ, page 330.)

From flat, thin tin-foil cut thirty-two wedge-shaped pieces four inches long. They should be one inch and a half wide at one end and three-quarters of an inch at the other, as shown at Fig. 2 A. Give each plate of glass two thin coats of shellac on both sides; then lay one on the paper pattern (Fig. 1) so that the outside edge of the glass will lie on the largest circle. Place a weight at the middle of the glass to hold it in place; then make sixteen of the tin-foil sectors fast to the plate, using shellac as the sticking medium. But first give one side of each sector a thin coat of shellac, allowing it to dry; then give it another coat when applying it to the glass. The sectors are to be symmetrically arranged on the glass, using a line of the pattern as a centre for each piece (as shown at A in Fig. 1), and the fourteen and six inch circles as the outer and inner boundaries. Each piece, as it is applied, should be pressed down upon the glass, so that it will stick smoothly, without air bubbles or creases. A very good plan is to lay a piece of soft blotting-paper over the sector and drive it down with a small squeegee-roller such as is used in photography, taking care, however, not to shift the sector from its proper position. When all the sectors are on, the plate should appear as shown in Fig. 2. After the shellac, which holds the sectors to the glass, is dry, run a brush full of shellac around the inner and outer extremities of the tin-foil strips for half or three-quarters of an inch in from the ends. The shellac will hold the sectors firmly to the glass, and will slightly insulate them as well, thereby preventing the escape of electricity. Apply the remaining sectors to the other plate of glass in a similar manner; and as a result two disks of glass, with the applied strips, will be ready to mount in the frame.

A hole three-quarters of an inch in diameter should be made in each glass plate, so that a three-eighths spindle may pass through them and into the bosses, so as to keep them in proper line. It is preferable, however, not to bore these holes if bosses and accurately bushed holes can be made in the uprights of the frame which support these disks.

At a wood-working mill have two bosses made that will measure four inches in diameter at the large end, and one inch and a half at the small one. They should be of such length that when the plates and two bosses are arranged in line (to appear as shown in A A at Fig. 9) they will fill the entire space between the uprights B B. Near the small end a groove is turned in each boss, so that a round leather belt will fit in it, as shown in Fig. 3.

The base is made from pine, white-wood, cypress, or any other wood that is soft and easily worked. It is composed of two strips twenty-four inches long, three inches wide, and one inch and a quarter in thickness, and two cross-pieces fourteen inches long, three inches wide, and one inch and a half thick.

These are put together with glue and screws, and at both sides of the base notches are cut to accommodate the feet of the uprights. The uprights are seventeen inches high, three inches wide, and one inch and a half thick. The notch at the foot of each one is cut so that, when fitted in place, the foot of the upright will rest on a table on a line with the bottom of the end cross-pieces under each corner. At the foot of the uprights a piece of sheet rubber should be made fast, with glue or shellac, so that when in operation the machine will not move about or slide.

A groove is cut at one side of each upright six inches above the bottom, as shown at Fig. 4 A. In this groove the driving-wheel axles fit, and near the top holes are made in the uprights through which the spindles pass, which in turn support the bosses and glass disks.

At the middle of each cross-piece forming the ends of the base a one-inch hole, for the glass standard rods, is bored through the wood, as shown at Fig. 4 B B. After attaching the uprights to the base with glue and screws, and giving all the wood-work several successive coats of shellac, the frame will be ready for its mountings.

The driving-wheels are of wood seven-eighths of an inch thick and seven inches in diameter; they should be turned on a lathe and a groove cut in the edge so that a round leather belt will fit in it. These wheels are mounted on a wooden axle that can be made from a round curtain-pole, with a half-inch hole bored through its entire length. The axle is as long as the distance between uprights B B in Fig. 9. The wheels are to be arranged and glued fast to the axle, so that the grooves will line directly under those in the bosses, as shown in Fig. 9. A half-inch axle is driven through the hub, and at one end it is threaded and provided with two washers and nuts; or a square shoulder and one washer and nut may be used, so that a crank and handle may be held fast. Shellac should be put on the shaft to make it hold fast in the hub.

The complete apparatus of wheels, axle, hub, and handle is shown at Fig. 5, and in the frame this is so hung that the iron axle rests in the grooves cut in the uprights. To hold this in place two metal straps, as shown in Fig. 6, are made and screwed fast to the wood. When finally adjusted the driving-wheels should rotate freely whenever the crank is turned. The wooden bosses, Fig. 3, are given two or three coats of shellac; then they are made fast to the glass disks on the same side to which the tin-foil sectors are attached. The disks should be placed over the paper plan, Fig. 1, and so adjusted that the outer line tallies with the large circle. Acetic glue[4] is then applied to the flat surface of the boss, and the latter is placed at the middle of the disk to line with the small circle. Place a weight on the end of the boss to hold it down, and leave it for ten or twelve hours or until thoroughly dry.

[4] See Formulæ, Chapter xiv., for the recipe of acetic glue.

Both bosses should be set at the same time so that they may dry together.

If the bosses are turned on a lathe a hole should be made in each one about half-way through from the small end. This, in turn, should be bushed or lined with a piece of brass tube, which should fit snugly in the hole. A little shellac painted on each piece of tube will make it stick. Pieces of steel rod that will just fit within the tubing are to be cut long enough to enter the full length of the hole, pass through the holes made in the top of the uprights, and extend half an inch beyond, as shown in Fig. 9. The bosses and axles will then appear as shown in Fig. 7.

Flat places should be filed on each rod where it passes through the wood upright; a set-screw will then hold it fast and keep it from revolving. When the hole, or tubing, is oiled so that the boss and disk will revolve freely on the axle, the disks, bosses, and axles are ready to be mounted in the frame.

A red fibre washer, such as is used in faucets, should be made fast to one glass disk at the centre, so as to separate the disks and prevent them from touching when they are revolving in opposite directions. These fibre washers can be had from a plumber or purchased at a hardware store. Shellac or acetic glue will hold the washers in place.

Mount one disk by holding the boss with the small end opposite a hole in one upright, and slip an axle through from the outside of the upright. Hold the other disk in place, and slip the remaining axle through the other upright and into the boss. When both plates are in place and centred, turn the set-screws down on the flattened axles to hold them in place.

To reduce the friction between the bosses and the uprights it would be well to place a fibre washer between them. A few drops of oil on these washers will lubricate them properly, and allow the machine to run easier. An end view of the apparatus, as so far assembled, will appear as shown in Fig. 9, A being the disks, bosses, and axles, B B the uprights supporting them, C the hub, and D D the driving-wheels. The handle and crank (E) extends out far enough from the side to allow a free swinging motion without hitting the upright or base. Having arranged these disks and wheels so as to revolve freely, it will now be necessary to construct and add the other vital parts and the connecting links in the chain of the complete working mechanism.

From a supply-house obtain two solid glass rods an inch in diameter and fifteen inches long. These fit in the holes (B B) bored in the end-pieces of the base, Fig. 4. Procure two brass balls, two or two and a half inches in diameter, such as come on brass beds, and two short pieces of brass tubing, one inch inside diameter, that will fit over the ends of the rods. These tubings are to be soldered fast to the balls so that both tubes and balls will remain at the top of the glass rods.

From brass rod three-sixteenths or a quarter of an inch in diameter make two forks, as shown at Fig. 8, and solder small brass balls at the ends of the rods. The prongs of the fork are six inches long and the shank four inches in length. Along the inside of the forks small holes are bored, and brass wires, or “points,” are soldered fast. These extend out for half an inch from the rods, and are known as the “comb,” or collectors. The forks should be so far apart that when mounted with the glass disks revolving between them the points will not touch or scratch the tin-foil sectors, and yet be as close to them as possible. A hole should be bored in the brass balls, and the shank of the fork passed through and soldered in place, as shown in Fig. 10.

A three-eighth-inch hole is bored directly in the top of each brass ball to hold the quadrant rods, which extend over the top of the disks.

In the illustration of the complete machine (Fig. 12) the arrangement of the glass pillars, balls, combs, and quadrant rods is shown. The rods are three-eighths of an inch in diameter and are loose in the holes at the top of the balls, so that they can be moved or shifted about, according as to whether it is a left or a right handed person who may be turning the crank.

At the upper end of each rod a brass ball is soldered, one being three-quarters of an inch in diameter, the other two inches. The projecting ends of the forks should be provided with metal handles or brass balls, as shown in Fig. 12; these may be slipped over the end or soldered fast. Obtain two small brass balls with shanks, such as screw on iron bed-posts, and have the extending ends of the axles that support the bosses threaded, so that the balls will screw on them. Bore a quarter-inch hole through each ball, and slip a brass rod through it and solder it fast. Each end of these rods should be tipped with a bunch of tinsel or fine copper wires. These are the “neutralizers,” and the ends are curved so that the brushes of fine wires will just touch the disks when the latter are revolved, as shown in Fig. 12. The ball holding the rod is to be screwed fast to the axle; then the axle is pushed back into the boss and made fast in the head of the upright with the set-screw.

The rod-and-ball at the opposite side of the disks is arranged in a similar manner, but the rod points in an opposite direction to that on the first side. Cord or leather belts connect the driving-pulleys and bosses, the belt on one side running up straight over the boss and down again around the driving-pulley. The belt at the opposite side is crossed, so that the direction of the boss is reversed; and in this manner the disks are made to revolve in opposite directions, although the driving-pulleys are both going in the same direction.

A portion of the sectors are omitted in the illustration (Fig. 12) so that a better view of the working parts may be had. When the disks are revolving the accumulated electricity discharges from one ball to the other, above the plates, in the form of bright blue sparks sufficiently powerful to puncture cardboard if it is held midway between the balls.

A Large Leyden-jar

When experimenting with this machine it would be well to have one or more Leyden-jars to accumulate static charges. A large one of considerable capacity is easily made from a battery jar, tin-foil, brass rods and chain, and some other small parts.

Obtain a bluestone battery jar, and after heating it to drive all moisture from the surface, give it a coat of shellac inside and out. With tin-foil, set with shellac, cover the bottom and inside of the jar for two-thirds of its height from the bottom, as shown in Fig. 11. Cover the outside and bottom in a similar manner, and the same height from the bottom, and provide a cork, or wooden cap, for the top. If a large, flat cork cannot be had, then make a stopper by cutting two circular pieces of wood, each half an inch thick, the inner one to fit snugly within the jar, the other to lap over the edges a quarter of an inch all around. Fasten these pieces together with glue, as shown at Fig. 13, and give them several good coats of shellac. Make a small hole at the middle of this cap and pass a quarter-inch rod through it, leaving six inches above and below the cap. To the top of the rod solder a brass ball. At the foot a piece of brass chain is to be made fast, so that several links of it rest on the tin-foil at the bottom of the jar.

To charge a jar from the Wimshurst machine, stand the jar on a glass-legged stool, and connect a copper wire between one of the overhead balls on the machine and the ball at the top of the rod in the stopper of the jar. Make another wire fast to the other ball at the top of the machine, and place it under the jar so that the tin-foil on the bottom touches it. By operating the machine the jar is charged.

To discharge the jar make a [T]-yoke, as shown at Fig. 14, by nailing a brass rod fast to a wooden handle and soldering brass knobs, or hammering a lead bullet, on the ends of the rod. Hold one knob against the top knob of the jar and bring the other near the foil coating at the outside, when a spark will jump from the foil to the knob with a loud snap.

A Glass-legged Stool

One of the most useful accessories in playing with frictional electricity will be a glass-legged stool. A stool with glass feet is perhaps too expensive for a boy to purchase, but one may be made at little or no cost from a piece of stout plank, four glass telegraph line-insulators, and the wooden screw-pins on which they rest when attached to a pole.

The general plan of the stool is shown at Fig. 15, and the top measures twelve by fifteen by two inches. Under each corner a screw-pin is made fast by boring a hole in the wood and setting the pin in glue. The pins are cut at the top, as shown in Fig. 16, and when they are set in place the glass insulators may be screwed on. The wood-work should be given a few coats of shellac to lend it a good appearance and help to insulate it.

There are a great many interesting experiments that may be performed with static or frictional electricity, and these may be looked up in the text-books on electricity used in school. A word of caution will not be misplaced. Remember that the current, in large volume, is dangerous. For example, a series of charged Leyden-jars may contain enough electricity to give a very severe shock to the nervous system of the person who chances to discharge it. Its medical use should be under the advice and supervision of a physician.