208 APPLIED SCIENCE 



sure may be varied. Circuit-breakers or fuses to interrupt 

 any particular circuit through which an excessive current 

 may flow, are also included in the switchboard equipment. 



A switchboard is always placed away from the wall or 

 ceiling to reduce the danger of communicating fire to adja- 

 cent combustible material. Conductors should be of soft 

 annealed copper, about 97% pure, and should be insulated 

 for their entire length by a vulcanized rubber compound 

 that adheres to the wire. Wires should be arranged to secure 

 distribution centers in easily accessible places so that cut- 

 outs and switches may be conveniently located. The load 

 should be divided as evenly as possible among all the branches, 

 and complicated and unnecessary wiring should be avoided. 



255. Transmission of Electrical Current. The electrical 

 current must be transmitted from the power plant to different 

 points of distribution in an economical manner; that is, 

 with very little loss of electricity, and at the same time in a 

 way that will reduce the danger to life to a minimum. The 

 problem is not serious when the generating plant is in the 

 same or an adjacent building, as in the case of a private 

 plant; but it is a serious problem in a central power plant 

 that supplies electricity over a large area. 



The current is usually transmitted, as noted above, through 

 copper wire supported on steel poles or towers, or in under- 

 ground conduits. The wire used underneath the ground 

 must be insulated, while the wire used overhead may or 

 may not be insulated. Overhead wires should be separated 

 as far as possible so they will not swing together. Over long 

 distances, such as 15 to 20 miles or more, the energy is trans- 

 mitted as alternating current at from 11,000 to 22,000 volts. 

 If the central station is near the center of distribution, the 



