Shadow Drawings
A very interesting entertainment is to be brought to a close by a short exhibition of shadow drawing. The apparatus required is a magic lantern, before the lens of which is fixed a prepared plate consisting of glass previously covered with a coating of lamp-black and water. A sheet of calico should be attached, some distance away, to the wall. Between the plate and the lantern, and a little to one side, stands the operator, who with the aid of a stick or stump sketches the subject upon the plate, the light filtering through throwing an enlarged presentment of the picture upon the calico screen.
CHAPTER XXXIV
FRICTIONAL FUN
Some Amusing Electrical Experiments[1]
Electricity may be said to have a finger in most pies. From its manifestations it would appear to be an all-pervading force, and modern research tends always to substantiate this conclusion. Thus two coins—a cent and a quarter—cannot be brought into contact without producing their quotum of electrical force, and in like manner the principles of its working may often be demonstrated by the simplest apparatus.
There are two states which this subtle force assumes, called Static and Dynamic. The former word indicates a condition of rest, and is applied to electricity when stored in suitable magazines ready for discharge at a favorable opportunity, just as water may be held in lofty reservoirs, ready to pour down and perform work—whether useful or otherwise—if the pipes be opened. Dynamic, on the other hand, indicates a state of motion; so that Dynamic Electricity means that it is flowing along conductors from one place, where there is a large quantity of the force, to another less powerful, in the same way as—again using the comparison with water—the latter pours along connecting pipes from a high level to a lower. Phenomena of static electricity are invariably produced by friction, and some experiments of this class will be first described.
The chief element of success in friction electrical experiments is a warm and dry atmosphere. The operations should therefore be conducted in a well-ventilated room where the fire has been burning some time, whilst all apparatus may with advantage have stood warming for some time before.
1. Rub a 6-inch square of brown paper with a warm silk handkerchief, then place flat against the wall or marble fireplace. It should cleave tightly to either of the latter surfaces.
2. Again electrify the brown paper and hold it above any willing person’s head of hair. The locks will fly up towards the paper and appear like bristles. You may then make sure of a safe retreat, and utter side remarks about “wire.”
3. Sprint smartly after the cat, which, like the other apparatus, should have been warming itself on the hearthrug, and having effected a capture, stroke the fur backwards. If the hand be not moist, considerable sparking should occur and be easily visible in a darkened room.
4. By combing dry hair quickly with a warm ebonite comb, and then applying this to the knuckles, distinct sparks may often be obtained.
5. Cut a piece of paper into small confetti, and place near it a stick of sealing-wax or ebonite rod, which has been rubbed with flannel. The paper atoms are attracted to the rod through a distance of one inch or more, and then, contact having once occurred, they immediately drop off.
6. In the previous experiment it was found that the paper atoms having once touched the rod, dropped off directly. This behavior was caused by a sequence of happenings.
First the negatively excited sealing-wax caused positive charges to possess the upper surface of the paper—the equal negative charges being driven downwards—so that, the two kinds of electric energy having a mutual affinity, the paper sprang towards the wax. Immediately they touched, however, the positive charges of the paper were neutralized by the large excess of negative energy from the sealing-wax, and then, since electricities of like sign exercise mutual repulsion, the negative paper was driven from the negative wax.
Now this repulsion of similarly charged particles is demonstrable by a straightforward experiment, whose simplicity of description is in nowise proportionate to its entertainment. In truth, it consists merely of blowing a soap-bubble and catching it upon an excited stick of glass or wax. The bubble becomes electrified, its myriads of particles repel one another, and the beautiful globe expands in size, possibly to breaking point.
7. Scratch any design or word upon a small sheet of indiarubber—such as is used in the manufacture of mats—and then dust over the surface a thoroughly-shaken mixture of red-lead and sulphur. When all excess of the powder has been blown away, the design should appear in grains of red-lead, showing to excellent effect on the gray rubber ground.
8. By the following method a spark of sufficient intensity to light house gas may be obtained from brown paper. Support a salver on four well-dried inverted tumblers, placing a rubber ring between the tray and each glass, and twist a copper wire round a spoon which rests on the tray. The other end of this wire must be held—insulated from the fingers by a piece of indiarubber or by the ordinary covering of the wire—just over the gas-jet, whilst about 1⁄8-inch away is held another piece in metallic connection with the hand; that is, in reality, via the operator’s body with the earth.
Now, if the gas be turned on slightly and a piece of warm brown paper, electrified by stroking vigorously with a dry brush, be placed on the salver, a spark should pass across the 1⁄8-inch gap and ignite the escaping gas. If the spark does not pass at once, approach the wire extremities slightly closer together.
Fig. 1.—Raising paper by touching wooden lath with electrified sealing-wax.
9. The following experiment owes its origin to Gray—an early worker in the field of static electricity: Erect two dry tumblers with a piece of waxed cardboard between, after the fashion shown in [Fig. 1], and lay a lath on the top glass. Beneath one end of the lath spread small pieces of paper, over an ebonite or wood surface supported on another inverted glass. Then if a stick of sealing-wax, which has been vigorously rubbed with flannel, be approached to the other end of the lath, the particles of paper will fly backwards and forwards between the lath and the ebonite surface—on which they would normally rest.
10. A simple trick, but one nevertheless causing considerable amusement to spectators, may be arranged in the following manner: Bore a hole carefully at the middle of a wooden meat skewer (A, [Fig. 2]), of such size as to take a bone knitting-needle tightly (B, [Fig. 2]).
Sharpen the blunt end of the skewer and mount it so as to turn easily between bearings erected on inverted wine-glasses. These may consist of two wood blocks (C, [Fig. 2]), in each of which a shallow conical hole is made to receive the points of the skewer. On these stand weights (E, [Fig. 2]) to keep wood-bearings firm. When this is in position and swings easily, adjust the needle until it balances exactly. It should be at right angles to the skewer, and may be fixed in position, if necessary, with glue or sealing-wax.
Fig. 2.—The balanced disc.
Now cut two discs of white paper (D, [Fig. 2]) of a diameter about 1 inch less than the knitting-needle’s length, and, having printed across one the word “DONE” in large type, glue them—centers at the point where the needle and skewer intersect—on either side of this cross. Erect the apparatus as shown, so that the needle stands vertical, with the blank paper disc facing the audience, and then invite the latter to see if they can effect a movement without either touching or blowing the disc or creating any disturbance whatever. The conditions should preclude any person, except the canny operator, from attaining success. He, however, innocently takes a postcard (previously warmed and dried), tears it sharply in half, and presents one ragged edge to the projecting bone needle. The cardboard became electrified by tearing, and should exercise quite enough attraction on the needle to upset its nice balance, whereupon the other disc becomes visible, together with its notice “DONE.” The audience may take the ambiguous meaning according to their pleasure.
Few further experiments can be performed without the use of special apparatus. So far, we have dealt only with the most minute quantities of electricity, and if more striking effects are to be produced, our stores of energy must be increased.
Fortunately instruments of a fairly wide application may be rudely constructed at home, although, naturally, results must not be looked for comparable to those obtained with perfected apparatus. However, brief directions now follow for the manufacture of such simple instruments, and so, as commencement, let us appropriate two 3-lb. jam pots from which Leyden jars are to be evolved.
Fig. 3.—Leyden jar.