105. The Currents. It should be noted that the current from the cell does not get into the secondary coil. The coils are thoroughly insulated from each other. The secondary current is an induced one, its voltage depending upon the relative number of turns of wire there are in the two coils. (See Transformers.) The secondary current is an alternating one; that is, it flows in one direction for an instant and then immediately reverses its direction. The rapidity of the alternations depends upon the speed of the interrupter. Coils are made that give a secondary current with an enormous voltage; so high, in fact, that the spark will pass many inches, and otherwise act like those produced by static electric machines.
Fig. 105.
106. Uses of Induction Coils. Gas-jets can be lighted at a distance with the spark from a coil, by extending wires from the secondary coil to the jet. Powder can be fired at a distance, and other things performed, when a high voltage current is needed. Its use in medicine has been noted. It is largely used in telephone work. Of late, great use has been made of the secondary current in experiments with vacuum-tubes, X-ray work, etc.
CHAPTER XIV.
THE ELECTRIC TELEGRAPH, AND HOW IT SENDS MESSAGES.
107. The Complete Telegraph Line consists of several instruments, switches, etc., etc., but its essential parts are: The Line, or wire, which connects the different stations; the Transmitter or Key; the Receiver or Sounder, and the Battery or Dynamo.
108. The Line is made of strong copper, iron, or soft steel wire. To keep the current in the line it is insulated, generally upon poles, by glass insulators. For very short lines two wires can be used, the line wire and the return; but for long lines the earth is used as a return, a wire from each end being joined to large metal plates sunk in the earth.
