510 MAGNETISM AND ELECTRICITY the first of many which he made to show that an electromotive force is produced, resulting in an electric current in a closed circuit, when a conductor is moved through a magnetic field. Electromotive Forces Resulting in Electric Current May Be Induced by Moving a Coil of Wire through a Magnetic Field. An electromotive force is produced, resulting in an electric current, by causing a conductor to move through a magnetic field. It makes no difference whether the source of the magnetic field is a permanent magnet or an electromagnet. The same effect may be produced by moving the magnetic field so that the conductor cuts the lines of force. Fig. 246. The galvanometer shows that a current of electricity is produced when the magnet is moved up or down within the coil. The current flows only during the motion. The strength of the electromotive force, i.e., the voltage, depends upon the number of lines of force cut per second and upon the number of turns in the coil which is cutting the lines of force, while the amount of current flowing, i.e., amperage, depends upon the strength of the magnetic field. A coil of wire called an "earth inductor" may be moved through the earth's lines of force and thus produce an electromotive force resulting in an electric current if rotated about an axis properly directed in its relation to the earth's lines of force. Induction Coils Have Many Uses. An induction coil is used to increase the electromotive force (voltage). It consists of two coils, one called the "primary," having only a few turns, while the other, the "secondary," has many turns. Every additional turn in the coil increases the voltage proportionately because the lines of force are cut once for each turn. Because the voltage of the induced current depends upon the relative number of turns of wire in the secondary and primary, a considerable increase in voltage can be obtained with an induction coil. the In order to induce a current in the secondary, a method must be found to move the electromagnetic field.
ELECTROMAGNETIC INDUCTION 511 This may be accomplished by use of an automatic interrupter, which makes and breaks the current several times a second. The induction coil consists of a strong electromagnet, made up of a core surrounded by the primary coil, and a secondary coil usually wrapped around the primary coil. Induction coils are capable of building up such high voltages that sparks may be made to jump several inches through the air. The distance a spark will jump through the air depends upon the dryness of the air, the shape of the conductors, the form of gap terminals, and other factors. In ordinary air an electromotive force of ten to thirty thousand volts is needed to make a spark jump across one centimeter. One can easily calculate the voltage required to make a spark jump several inches, using information given in a previous section. Induction coils are used where high voltage and low amperage are needed, as in furnishing a spark for igniting gases in internal-combustion engines or in operating X-ray tubes. The Tesla coil is a type of induction coil, designed by an American electrician, Nicola Tesla, for the production of very high voltage. It depends on the use of a very rapidly oscillating current in a coil, which greatly increases the rate at which the magnetic field changes. This oscillating current, sometimes oscillating with a frequency of a million times a second, is produced by the alternate charging and discharging of a condenser. An induction coil is used to charge the condenser. With the Tesla coil many spectacular experiments can be carried out. Although the voltage is very high and produces vivid flamelike discharges, it is quite harmless. One is not hurt by touching one of the terminals, because the current generated is very small and can do no harm because of the rapidity of the oscillations, and because the currents do not penetrate to vital organs. Transformers Are Induction Coils Operated by Alternating Currents. Induction coils require interrupters inasmuch as they use the direct current. Transformers are induction coils used to change the voltage when employing alternating current. Inasmuch as the direction of the alternating current changes many times per second, no interrupters are required in transformers. For many purposes direct current is required, but alternating current can frequently be used and has supplanted the direct type in most communities because electric power can be transmitted more economically by alternating current. The reason for this is that the voltage in the case of an alternating current can be raised by a transformer to a point where the loss in transmission due to resistance is reduced to a minimum; the high voltage can be lowered by another transformer at the place where the current is to be