Turning the handle of the machine rapidly for ten or fifteen seconds will charge the jar. Disconnect the wires as promptly as possible so that the electricity in the jar will not have a chance to leak back into the machine. Be very careful while doing this, however, because if you should happen to touch the tinfoil on the outside of the jar and the rod which connects with the inside coating at the same time you will get one of the surprises of your life.
[Illustration: FIG. 16.—A Large Leyden Jar for experimental purposes.]
The shock won’t really hurt you any but it will be very uncomfortable and somewhat surprising.
You can discharge a Leyden jar by bringing a piece of wire which is connected to the outside coating, near to the knob on the rod. When the wire is close to the ball the electricity will jump across the space in the shape of a snapping white spark.
The Leyden jar can be used in connection with a number of experiments described later on.
[Illustration: FIG. 17.—Showing how to Discharge a Leyden Jar with a curved piece of stiff wire fitted to a Wooden Handle.]
*Bottled Lightning.* A very pretty effect can be obtained by passing the spark from a Leyden jar or a static machine over a "lightning board." A "lightning board" consists of a pane of glass having a number of small squares of tinfoil stuck on it so that when the electrical discharge is passed over it, sparks take place between the little tinfoil squares and produce an effect something like miniature lightning.
A lightning board suitable for the static machine just described may be made from a strip of ordinary window glass about nine inches long and two inches wide.
Clean the glass thoroughly and then give it a coat of shellac on one side. As soon as the shellac becomes sticky, lay on a strip of tinfoil the same size as the glass and rub it down smoothly. When the shellac has thoroughly dried so that the tinfoil is stuck tightly to the glass, the board is ready to be cut up into squares. This can be best accomplished by means of a sharp knife and a ruler. Use care in doing the work so as not to tear the tinfoil and be sure that the knife cuts all the way through to the glass. Leave two solid strips of tinfoil at each end to which to make connections.
[Illustration: FIG. 18.—The "Lightning Board" is simply a Strip of Glass covered with small Tinfoil Squares. It may be insulated by mounting on a Bottle. The two Wires attached to the wide Tinfoil Strips at the ends of the "Board" are for connection to the Static Machine or Leyden Jar.]