20 nature's 



ger, but it would be exceedingly dangerous to 

 be put in contact with a pressure of 1000 or 

 more volts, such as is used for lighting pur- 

 poses. 



In principle the transformer is nothing 

 more or less than an induction-coil on a very 

 large scale. The ordinary induction-coil, such 

 as is used for medical purposes, is ordinarily 

 constructed by winding a coarse wire around 

 an iron core. This core is usually made of a 

 bundle of soft iron wires, because the wires 

 more readily magnetize and demagnetize than 

 a solid iron core would. Around this coil of 

 coarse wire, which we call the primary coil, is 

 wound a secondary coil of finer wire. If now 

 a battery is connected with the primary coil, 

 which is made of the coarse wire, and the cir- 

 cuit is interrupted by some sort of mechan- 

 ical circuit-breaker, each time the primary or 

 battery circuit is opened there will be a mo- 

 mentary impulse in the secondary circuit of a 

 much higher voltage; and at the moment the 

 primary circuit is closed there will be another 

 impulse in this secondary circuit in the oppo- 

 site direction. The latter impulse is called 

 the initial and the former the terminal im- 

 pulse. A current created in this manner is 

 called an induced current. The initial current 

 is not so strong as the terminal in this par- 

 ticular arrangement. 



If we should take hold of the two wires con- 



