291 

 A Standard for the Measurement of High Voltages. 



C. Francis Harding. 



Modern developments in the generation, transmission, distribution and 

 utilization of electricity at high voltages have greatly outstripped the accurate 

 measurement of such voltages. Those familiar with the very accurate stand- 

 ards and measurements of voltage, current and power at low potentials may 

 be surprised to learn that the recognized standard for the determination 

 of high voltages is the needle or sphere spark gap. In other words the voltage 

 if measured simply by the distance that it will cause a spark to jump in air 

 between needle points or spheres under specified conditions. 



It is hardly necessary to point out that such a standard is readily affected 

 by temperature, humidity and barometric changes, not to mention the 

 presence of other conductors which may be in the immediate vicinity. It 

 is therefore not readily reproducible and it is most difficult to make the two 

 standards agree at 50 kilovolts at which voltage both should be accurate. 



With these facts in mind, an attempt is being made in the electrical 

 laboratories of Purdue University to develop a more satisfactory standard 

 for the measurement of high voltages which is based upon the fundamental 

 principles of the electrostatic field. Although many forms of electrostatic- 

 voltmeters have been developed in the past, in the endeavor to commercialize 

 them and make them compact, the very uniform field upon which their 

 accuracy depends has been sacrificed. Xo attempt has been made to make 

 the standard voltmeter described herein portable or a thing of beauty, for it 

 is believed that such qualities are quite subordinate m the consideration 

 of a primary standard. 



If a perfectly uniform electrostatic field is produced between two parallel 

 metal plates it can be readily shown that the force action between such 

 plates expressed in dynes is 

 AE 2 K 



P = 



8ttU 

 where A = area of plate in square centimeters 



E = potential expressed in electrostatic units 

 K = dielectric constant ("unity for air) 

 t = distance between plates in centimeters. 



