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- 



793 



