SURGING CHARACTER OF HIGH-TENSION CURRENTS.--When a current of electricity is sent through a wire, hundreds of miles in length, the current surges back and forth on the wire many thousands of times a second. Light comes to us from the sun, over 90,000,000 of miles, through the ether. It is as reasonable to suppose, or infer, that the ether can, therefore, convey an electrical impulse as readily as does a wire. It is on this principle that impulses are sent for thousands of miles, and no doubt they extend even farther, if the proper mechanism could be devised to detect movement of the waves so propagated. THE COHERER.--The instrument for detecting these impulses, or disturbances, in the ether is generally called a _coherer_, although detector is the term which is most satisfactory. The name coherer comes from the first practical instrument made for this purpose. [Illustration: _Fig. 75._ WIRELESS TELEGRAPHY COHERER] HOW MADE.--The coherer is simply a tube, say, of glass, within which is placed iron filings. When the oscillations surge through the secondary coil the pressure or potentiality of the current finally causes it to leap across the small space separating the filings and, as it were, it welds together their edges so that a current freely passes. The bringing together of the particles, under these conditions, is called cohering. Fig. 75 shows the simplest form of coherer. The posts (A) are firmly affixed to the base (B), each post having an adjusting screw (C) in its upper end, and these screw downwardly against and serve to bind a pair of horizontal rods (D), the inner ends of which closely approach each other. These may be adjusted so as to be as near together or as far apart as desired. E is a glass tube in which the ends of the rods (D) rest, and between the separated ends of the rods (D) the iron filings (F) are placed. THE DECOHERERS.--For the purpose of causing the metal filings to fall apart, or decohere, the tube is tapped lightly, and this is done by a little object like the clapper of an electric bell. In practice, the coils and the parts directly connected with it are put together on one base. THE SENDING APPARATUS.--Fig. 76 shows a section of a coil with its connection in the sending station. The spark gap rods (A) may be swung so as to bring them closer together or farther apart, but they must not at any time contact with each other. The induction coil has one terminal of the primary coil connected up by a wire (B) with one post of a telegraph key, and the other post of the key has a wire connection (C), with one side of a storage battery. The other side of the battery has a wire (D) running to the other terminal of the primary. [Illustration: _Fig. 76._ WIRELESS SENDING APPARATUS]