TRANSMISSION OF ELECTRICAL ENERGY 209 



voltage is about 2200 volts, and is reduced by transformers 

 before it reaches the consumer. 



Alternating current is usually generated at a medium volt- 

 age and then raised by step-up transformers for transmission 

 purposes. When the current of high voltage reaches the 

 substation, it is reduced by means of step-down transformers. 

 If necessary, the alternating current may be changed over 

 to direct current by means of a rotary converter. 



Electrical energy must be furnished to meet the maximum 

 demand during any part of the day, even if this maximum 

 demand continues only for a short time. To avoid the 

 expense and large investment of an equipment big enough 

 to supply such a maximum, storage batteries are utilized to 

 store up current during the slack hours and distribute it 

 during the rush hours of the evening when many lights are 

 burning. In this way the equipment is kept evenly at work 

 throughout the day. 



256. Measurement of Strength of Current. Electricity 

 is distributed from the power station where the energy is 

 generated to the different points where it is to be utilized for 

 power or lighting. The amount of work done or " power" 

 consumed in transmitting electricity from the power station 

 to the point of consumption is found in the following manner : 

 Multiply the electromotive force, determined by the volt- 

 meter, and the strength of the current, determined by the 

 ammeter, and the time in seconds; the result is the power 

 consumed and is expressed in joules, the electrical unit of 

 energy. This formula may be written: 



Energy = Pressure X quantity X time 

 Joules = Volts X amperes X seconds 



