All you have to do to make your induction coil into a wireless transmitter, that is, the sending apparatus, is to put a couple of brass balls on the points of the spark-gap, fasten a wire to one of them and the other end to a nail in the wall near the ceiling and then connect the other one with a wire which ends in a small sheet of brass or copper that rests on the floor as shown at A in Fig. 123.

To make a receiver that will tap out the signals you send on your transmitter, you will need (a) a coherer, (b) a relay, (c) an electric bell and (d) a dry cell. You can make the coherer but the other three pieces of the apparatus you had better buy.

Fig. 123. A Simple Wireless Demonstration Set

For the coherer cut off a piece of brass rod 1/8 inch in diameter and 1 1/4 inches long, file the ends off even and slip them through the holes in the binding post. Put a pinch of nickel and silver filings into a piece of glass tubing about an inch long and push the ends of the rod into the tube with the filings between them.

Screw the rods into a couple of binding posts set 2 inches apart on a block as shown at B and your coherer is done.

Connect up the coherer, relay, tapper and dry cell on a board as shown in the wiring diagram at C; fasten a wire to one of the rods of the coherer and to a nail near the ceiling; fix a wire to the other coherer rod and to a small sheet of brass or copper which rests on the floor.

Fig. 123B. cross section of the coherer showing its Construction

Now when you press the key or button of the sender, which is on one side of the room, the bell of the receiver, which is on the opposite side of the room, will ring out a signal. The fact that there are no wires connecting the sender with the receiver will create much wonder.

The theory of wireless telegraphy is rather deep but you will find it simply explained in my Book of Wireless published by D. Appleton and Co., New York City.