HIGH VOLTAGE COMPTON 



263 



tho belt on its return journey, thus doubling the current output. A 

 second refinement consisted of a self-exciting charging device 

 Avhereby the entire generator could be made to operate independently 

 of any external source of electricity. Not only does tliis device attain 

 the desired result in what appears to be the simplest possible manner, 

 but it is also interesting to note that the energy transformations in 

 its operations are exceedingly simple, consisting oidy in the trans- 

 formation of the energy required to drive the belt into work done in 

 separating and transfci-i'ing electric ch:irge from earth potential to 

 sphere potential. Figure 5 shows, schematically, the operation of 

 this aenerator. 



{E>f?TH 



FiGDRB 5. — Schematic diagram of Van de Graaflf electrostatic generator. 



By this means electricity is continually conveyed to the spherical 

 terminal, whose potential consequently rises until limited by the 

 breakdown of the insulation of the air in the form of a corona 

 discharge at the surface of the spheres. This breakdown voltage 

 depends on the size of the sphere, being approximately 750,000 volts 

 for a 2-foot sphere and increasing to 5 million volts for a 15-foot 



