HIGH VOLTAGE — COMPTON 253 



order and comes out of that secondary as a very large current at 

 relatively small voltage. 



There has been no really fundamental difficulty to be overcome in 

 these power transmission lines, although there have been very many 

 interesting problems of science and engineering to be solved. The 

 proper design of a transformer to be efficient and to be sufficiently 

 well insulated is one problem. Perhaps the most difficult problem has 

 been that of proper switching devices so that these high-voltage cur- 

 rents can be started and stopped without excessive arcing at the 

 switches. It is such developments as these which have made the great 

 generating stations at Niagara Falls and the many other hydro- 

 electric or steam-electric generating stations such an important 

 feature in our present industrial life. 



The second thing that has stimulated high-voltage developments 

 of the electromagnetic type has been the X-ray. For ordinary pur- 

 poses, from 30,000 to 100,000 volts are adequate for either diagnostic 

 or therapeutic purposes. Of recent years, however, in the endeavor 

 to find the most effective methods of treating internal cancerous 

 growth there has been an increasing desire to go to much higher 

 voltages, and consequently X-ray tubes operating on as much as a 

 million volts have been developed at the General Electric Co., at the 

 California Institute of Technology, and elsewhere. 



To generate the high-voltage power for tliese X-ray tubes, recourse 

 has been had to transformers connected in series, the primary of one 

 transformer being connected with the secondary of the other, and all 

 transformers after the first being insulated. By such means, large 

 power can be delivered and high voltages obtained, although a million 

 volts appears to be about the practical limit, because there are para- 

 sitic currents known as " charging currents " which drain a great deal 

 of energy uselessly from the system when an alternating or " varying 

 current " is used at such high voltages. Furthermore, the equipment 

 becomes tremendously expensive on account of the requirements for 

 insulation. 



The third thing that has led to high-voltage developments of the 

 electromagnetic induction type has been the study of the effect of 

 lightning on transmission lines and the desire of electrical engineers 

 to duplicate as nearly as possible the effect of lightning by means of 

 high-voltage sources for laboratory study. For this purpose there 

 has been developed the impulse generator, in which a series of con- 

 densers capable of storing electric charge at high voltage are charged 

 in parallel from a high-voltage transformer and are then connected 

 in series, so that the overall voltage delivered is the sum of the volt- 

 ages across the separate condensers. By such means impulsive or 

 momentary voltages of 10 or 15 million volts have been obtained. 



