238 ELEMENTS OF GENERAL SCIENCE 



instrument transformers, spark coils, induction coils, motor- 

 starting transformers, etc. the list is almost without end. 



244. The supplying of energy to the home. How does the 

 transformer enable the dynamo to supply energy for lamps, 

 heating devices, and motors ? Three important facts should 

 be kept in mind : first, electricity is not consumed hi the 

 home, as is coal or gas, but returns to the generator ; second, 

 electricity, like moving water, is only a means of transferring 

 energy, and the power company sells the energy possessed by 

 the moving electricity ; third, a transformer such as we have 

 described is used only on an alternating-current circuit. 



The direct-current power plant is used chiefly where the 

 energy is not transferred to a great distance, as in rural com- 

 munities, villages, homes, and stores, and often in cities for 

 hotels, schools, and factories. The rural plant usually con- 

 sists of a small direct-current generator having a capacity of 

 about 1000 watts at 32 to 40 volts, a storage battery con- 

 sisting of from 16 to 20 cells connected in series, a switchboard 

 for the instruments, fuses, etc., and a gasoline engine to drive 

 the generator. The larger direct-current generators used in 

 factories and sometimes for the central power stations in 

 small towns operate at from 250 to 300 volts or more. The 

 current is obtained direct from the generator mams, which 

 usually consist of large, expensive copper cables. 



245. Use of direct and alternating currents. It is greatly to 

 the disadvantage of the direct-current system that large con- 

 ductors are required. The energy transmitted is determined 

 by the current and the voltage. The number of watts will 

 be equal to the product of the amperes and the volts ; hence, 

 when the voltage is low, as it must be in a machine having 

 a commutator, the current must be high in order to transmit 

 a definite number of kilowatts. The heat losses in a wire 

 increase with the square of the current ; that is, to double 

 the current makes the heating effect four times as great. 

 The difficulty is solved, however, if the voltage is increased 



