Demonstrating Wireless Telegraphy.
—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 1¹⁄₄ 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