Our next step was to put antennæ upon the spark coil also, as shown in [Fig. 185]. One of these wires was stretched out upon the floor, while the other one was connected with the wire of a picture hanging upon the wall on the opposite side of the room from where the coherer was. We now found that the coherer would respond when the spark coil was operated in the farthest part of the room. With the wires which were lying upon the floor extending toward each other, but lacking several feet of touching, the coherer responded when the spark coil was operated in various other rooms of the house, although the doors between were shut. When the floor wires were connected to the water pipes the coherer would respond when the spark coil was operated in a neighbouring house. We tried a similar experiment, substituting an ordinary electric bell for the spark coil. The coherer or electric eye detected that ether waves were sent forth from an electric bell every time a spark was produced in the bell. For this purpose connections were made, as shown in [Fig. 186]. One dry battery cell was used to ring the bell. The floor wire a, or, as it is usually called, the ground wire, was connected to the binding post 1, and the other antenna was connected to the screw 3, and then supported aloft on a picture hung upon the wall. With this transmitter we sent waves across the room which were detected by the coherer.
Fig. 187
We constructed a simple spark coil as follows: We bought a pound of No. 24 single cotton covered copper wire, such as is used in the electro-magnets of bells. It was, when we bought it, wound upon a wooden spool. We filled the hole in the centre of this spool with wire nails. One dry cell was connected with this ([Fig. 187]). When the wires at d were touched together, and then separated, a spark was produced at that point. A ground wire was connected at b, and an antenna at c, as before. Using this apparatus now as a transmitter of ether waves, we found that the coherer detected them.
Fig. 188
We next gave our attention to making changes in the receiving apparatus, not to change the coherer, but to provide substitutes for the ammeter. A sensitive relay was procured, which is essentially like a bell or buzzer except that it does not clatter. It will be readily understood, by referring to the accompanying [Fig. 188], that R is a coil of insulated wire around an iron core exactly like what we see in the electric bell. (In practice there will be a pair instead of one of them.) Such coils are called electro-magnets, because when electricity flows in the wires they become magnets, and will attract iron. A is an iron spring, B is a dry battery cell and C is the coherer. Whenever an ether wave passes the coherer permits the battery cell to send a current around the magnet of the relay, and it attracts the iron spring a, so that it hits against the metal post d with a click. Whenever we used this to respond to ether waves the click of the relay suggested the telegraph sounder. How it served in wireless telegraphy will appear in the following pages.