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. [Illustration: Fig. 187] 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. [Illustration: Fig. 188] XXII RINGING BELLS AND LIGHTING LAMPS BY ELECTRIC WAVES [Illustration: Fig. 189]