Page:Popular Science Monthly Volume 92.djvu/809

 Electrical Devices and How They Work

v.— Principles of the induction coil and transformer By Peter J. M. Clute, B. E.

��IF a coil of insulated wire is wound around an iron core, as shown in Fig, 1, and connected to a battery circuit, and if another coil is wrapped about the same core and its terminals connected to any current detector, as shown in the illustration, it will be found that when the key is closed, the deflection

���Illustrating the working of an induction coil and alternating current transformer

of the detector needle indicates a tempo- rary current induced in one direction through the left coil. However, when the key is released, an equal but opposite deflection will be an indication of an equal induced current in the opposite direction.

This simple experiment illustrates the fundamental principle of the induction coil and the alternating-current trans- former. The right coil which is connected to the source of current, is called the primary coil, and the left coil, in which current is induced, is the secondary coil. This coil causes lines of force to exist inside of the primary coil — in other words, magnetizes the space inside of left coil, which is the core about which both coils are wound — and thereby causes an induced current to flow in left coil. De- magnetizing the space inside of left coil also induces a current in the coil. This is in accordance with Lenz's Law, namely, that any change in the number of mag- netic lines of force which thread through a coil induces a current in the coil.

If half of the turns of the secondary are unwrapped, the deflection when the cir- cuit is opened or closed will be found to be about half as great as before. Since the resistance of the circuit has not

��changed, it can be deduced that the E. M. F. of the secondary is proportional to the number of turns of wire upon it. This results from the principle that the E. M. F. induced in any circuit is equal to the rate of cutting of lines of force by that circuit. All the lines produced by the primary and which pass through the core, cut all the secondary turns. If, therefore, there are twice as many turns in one case as in another, theoretically twice as many lines of force cut the circuit, and hence the E. M. F. is twice as great. If, then, it is desired to obtain a very high secondary voltage, it is only necessary to build the secondary coil of a very large number of turns of fine insu- lated wire.

The induction coil, shown diagram- matically in Fig. 2, consists of an iron core C, composed of a bundle of soft iron wires; a primary coil wrapped around this core and consisting of a small number of turns of coarse insulated copper wire, connected to the battery circuit through the contact-point at the end of the screw D; a secondary S surrounding the

���A diagrammatic illustration of an induction coil with one wire coil on top of the other

primary is indicated, and consisting of a very large number of turns of fine copper wire, the terminals of which are t and t'; and an electromagnetic hammer H, or other arrangement for making and break- ing the primary circuit.

When the primary is closed, the core becomes magnetized. Thereupon, the iron hammer H is drawn away by mag-

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