802 The Outline of Science 



direct current to feed the live rails of various underground and 

 electric railways and tramways. 



Distribution of Current 



Current is usually generated in its alternating form, at a 

 pressure of 3.000 volts upwards. A volt, it should be explained, 

 is the electrical counterpart of the "pound per square inch" used 

 to describe the pressure of gases or liquids. To one side of the 

 power-house, perhaps in a separate chamber, will be found the 

 switchboard on which are mounted electrical gauges for measur- 

 ing the condition of current, and handles for operating the 

 switches whereby current is controlled and directed. 



Where the generators are many in number, they may be 

 divided into groups, each having its own switchboard. Just as 

 the units of a battery of boilers contribute through their individual 

 branch pipes and valves to a main steam pipe, so the generators of 

 a group feed a set of short collecting conductors, called 'bus bars, 

 through switches on the switchboard. 



By means of other switches the 'bus bars can be linked with 

 one or other of the sets of conductors which carry the current away 

 from the power-house for distribution; and, where there are 

 several groups of generators, provision is made for cross-con- 

 necting the several groups enabling them to come to one another's 

 aid when necessary. The switchboards collectively may be com- 

 pared to the bridge of a ship ; they are the centre from which the 

 brain and hand of the operator direct the mighty forces seeking an 

 outlet. The movement of a lever may send current representing 

 thousands of horse-power to a point within sight of the power- 

 house, or start it on a journey of hundreds of miles. 



Current intended for local distribution may be turned into 

 the mains straight from the generators, to be modified subse- 

 quently as will be described later. On the other 'hand, it may 

 first be sent to transformers, by which its voltage or pressure is 

 increased or stepped-up, as electricians say. The loss in trans- 



