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.
Leyden Jars
Procure as much metal foil as possible, either by raiding the nursery or by purchasing some tinfoil-wrapped butter scotch at the confectioner’s, and gum it round the jam jars so as to extend about half-way up the outside (A, [Fig. 3]).
Next solder stiff brass wires (8 inches long) to two round pieces of metal, so cut as to pass the jar mouths readily. To the other end of one brass rod solder another disc of metal, tin—or, better, brass—nicely smoothed along its two edges by sandpaper. The other brass rod may be looped at its end and have tinfoil wrapped round until a respectable knob be formed, or else have a brass bedstead globe soldered on.
