Fig. 184
The spark coil sent electric waves out in every direction, and those which hit the metal filings made them cohere together. In that condition they allowed the dry cell to send through them enough current to move the needle of the ammeter. Tapping the tube made the metal filings break apart again, in which condition they do not allow the current of the cell to pass in sufficient quantity to move the needle. This tube is called a coherer, because the filings in it cohere together. The apparatus then serves to indicate when electric waves are passing. As yet, however, it would not respond when the spark coil was more than one foot away. Our next step was to attach extra pieces of wire, each ten or twelve feet long, at either end of the coherer, as indicated in [Fig. 184]. 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.
We now found that the coherer would respond when the spark coil was operated several feet away. The extra wires which we had attached to the coherer are called antennæ, because they suggest the long "feelers" or antennæ of some insects.
Fig. 185
Fig. 186