6 CONSTANT-VOLTAGE TRANSMISSION 



applications usually have a voltage between conductors 

 of no or 220 volts, since such a voltage is suitable for 

 lamps, small motors, and many appliances. If the dis- 

 tance is increased, it is found that the voltage used up 

 in driving the current through the long wires becomes a 

 noticeable proportion of the working voltage, which is 

 thereby subjected to troublesome variations, as described 

 more fully in Chapter V. This trouble could be counter- 

 acted by using very large conductors, but it is cheaper 

 to apply a higher voltage, such as 2,200 volts, to the cir- 

 cuit, and then transform to the low voltage where 

 needed. With this arrangement, the voltage drop is a 

 small percentage of the higher line voltage, and its actual 

 value is reduced, since the current is less for a certain 

 amount of power at the high voltage. It was early found 

 in the development of electrical machinery that the use 

 of high voltages was a very successful means of obtaining 

 economical transmission of energy to long distances, and 

 within a few years the voltages used had reached many 

 thousand, and the distances traversed had extended to 

 many miles. 



It is obvious that for the longest distances, use must 

 be made of the highest voltages in order to keep the 

 current small and the voltage variation within reasonable 

 limits. The voltage used for ordinary cases can be ap- 

 proximated by the rough rule of 1,000 volts per mile of 

 distance of transmission. This represents the choice of a 

 voltage giving a balance between the extra cost of heavy 

 conductors when the voltage is low and the extra cost 

 of transformers and circuit breakers, and of generally 

 superior construction, when the voltage is high. 



When the voltage is much over 110,000 volts, the cost 



