252 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1933 



suits that he gave little thought to the possible practical application 

 of his work. It is said that when he was once urged to press his 

 claim as inventor of the telegraph and other instruments, he replied 

 that there were far too many interesting things to be done in the 

 laboratory to permit him to take time with such matters. 



There has been a practical urge for the development of high 

 voltage power from three different points of view. The first and 

 most important of these is for the transmission of electric power 

 over large distances. It is much more economical to transmit power 

 at high voltage and small current than at low voltage with large 

 current, because the resistance losses depend upon the current and 

 not the voltage. For this reason the voltage of high-power trans- 

 mission lines has continually risen from first a few hundred volts, 

 then a few thousand, not many years ago 60,000, and now upwards 

 of 200,000 volts. The losses of power and the necessity of auxiliary 

 equipment are such that, according to a practical rule, it is not 

 economical to transmit electrical power farther than 1 mile for every 

 1,000 volts. From this we see that a modern 200,000-volt trans- 

 mission line could be economically used to transfer power from 

 the power-generating station to distances of about 200 miles, but 

 beyond these distances such transmission of power is not economical. 

 For that reason, in any area requiring the use of electricity, power- 

 generating stations must be located at distances of not more than 

 200 miles from each other. 



The question may be asked as to why the voltage is not raised still 

 higher than 200,000 volts; and the answer to this is found in the 

 fact that, with higher voltages, the electric field in the air surround- 

 ing the wire becomes so intense as to ionize the air, causing a leak- 

 age of electricity from the wire into the air in the form of an electric 

 discharge known as a " corona." This phenomenon of corona is one 

 of the facts which set the practical upper limit to the voltage which 

 can be used for transmission. 



It is not feasible to generate directly voltages in the range of 

 several hundred thousand volts, because the difficulty of insulation 

 becomes too great, and an electric dynamo with insulation adequate 

 to withstand even several thousand volts would have to be so large, 

 to include the necessary insulation, as to be unwieldy and inefficient. 

 Consequently, the power is generated at relatively low voltage, 

 usually a few hundred volts in alternating current, and this is sent 

 through a step-up transformer insulated in oil, in which the second- 

 ary has a hundred or a thousand times more turns of wire than the 

 primary. In this secondary coil the very high voltage is generated 

 for transmission over the power lines. Then at the other end of the 

 line the power is fed through a similar transformer in the reverse 



