For detecting the electric waves which may be sent out from the spark gap of our automobile spark coil I shall ask you to help me prepare a special piece of apparatus. One boy may file this silver ten-cent piece and another may file this nickel five-cent piece, each gathering the filings upon a piece of paper. A third boy may select a piece of glass tubing about one eighth of an inch in the inside diameter, and with a three-cornered file cut off a short piece, about one and a half inches long, and smooth the ends with a wet file. A fourth boy may select a piece of stout copper wire nearly as large as the bore of the glass tubing, and cut from it two pieces, each about two inches long. Wind one end of each of these with thread to make them fit snugly in the glass tubing. [Illustration: Fig. 182 Coherer] We thrust one of the wires into the tube, then mixed equal parts of the silver and nickel filings and put as much of the mixture into the tube as we could hold upon the tip of a penknife blade, and then thrust in the other copper wire. (See Fig. 182.) The ends of the wire were about one eighth of an inch apart and the gap was loosely filled with the metal filings. This was connected by short pieces of copper wire, as shown in Fig. 183, to a dry battery cell, _B_, and a sensitive ammeter. When all connections were made the needle of the ammeter remained at zero, showing that no electric current was passing, that is, the battery cell was unable to send any electricity through the metal filings. This is the apparatus which is to help us detect electric waves when they pass about us. Electricity has been called invisible light, that is, invisible to our eyes, and this apparatus has been called an "electric eye" because it will detect electric waves in the ether, just as our eyes may detect light waves passing through the ether. [Illustration: Fig. 183] We placed the automobile spark coil upon the table near to the tube containing the filings of silver and nickel, and as soon as we made a spark pass between the knobs the ammeter needle moved half way across the scale, indicating that the spark had somehow influenced the metal filings in the tube so that now they permitted the battery cell to send some electric current through them and through the ammeter. I asked one of the boys to tap the tube slightly with a lead pencil so as to jar the metal filings, and as soon as he did so the needle of the ammeter went back to zero. [Illustration: Fig. 184]