Transportation on Land 329 



pressure at 2500 or even 110 volts, at which it can be used with 

 safety. This change in pressure is made by means of the 

 transformer. 



The principle of the transformer may be suggested, though 

 details will have to be sought in treatises on electricity. The 

 principle is simple and may be illustrated in the diagram of Fig. 

 108. A core of soft iron is made to form a circle so as give a 

 complete circuit for the flow of the magnetic force. A small 

 insulated wire has forty turns 

 around the core, forming the first or 

 primary coil. A large insulated wire 

 has two turns around the core, and is 

 known as the secondary coil. The 

 feed wires coming from the generator 

 are connected with the ends of the 



Small wire Or primary Coil. The the primary or supply wire to 



service wires of a house, for example, two . coils . of the secondar y or 



* ' service wire upon a common 



are connected with the ends of the core transforms the high volt- 

 large wire or secondary coil. An al- ^ ^^ t ut one - twentieth of 

 ternating current of about 2300 volts 



passes along the wire of the primary coil, and there is produced 

 in the iron core a magnetic current which moves first in one 

 direction and then in the other (page 325). As the magnetic 

 current passes along the iron core through the secondary coil, 

 there is induced in this an alternating electric current of about 

 fo of the voltage received, or 110 volts. The pressure or volt- 

 age is thus transformed or stepped down to that which may 

 be used safely in the house for lighting and heating. The loss 

 of energy in the transformer is very small. 



The principle may be expressed as follows : " The volts of 

 an alternating current received in the primary coil are to the volts 

 delivered by the secondary as the number of turns of wire of the 

 primary is to the number of turns of wire of the secondary." If 

 there were no loss in the transformer, the electrical energy 

 delivered would be equal to that received. 



