262 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



given another push, and so on. The few-thousand-volts push is 

 given to the ion every time it crosses the gap. It proceeds in ever- 

 widening circles, attaining a speed limited onlj^ by the dimensions of 

 the apparatus. With this device, Lawrence and his colleagues have 

 reason for hoping that the speeds may ultimately be increased up to 

 perhaps the equivalent of 10 million volts. 



The currents are not very large, being reported of the order of a 

 thousandth of a microampere. Nevertheless, these currents are tre- 

 mendous in comparison with anything that can be obtained from 

 radioactive material, and this source of high-speed, electrified par- 

 ticles will evidently be an important tool in nuclear investigation, as 

 is in fact evident from very recent reports from Professor Lawrence's 

 laboratory in which the experiments of Cockcroft and Walton in 

 disintegrating lithium nuclei by means of high-speed protons have 

 been confirmed and extended. 



In the construction of this apparatus, the largest magnet ever built 

 in this country has been put into use. 



We come now to what I believe to be the most important develop- 

 ment that has ever taken place in the field of extremely high voltages, 

 namely, the Van de Graaff generator, invented by Dr. Van de Graaff 

 as a result of considerations which were developed while he was a 

 Rhodes scholar in England and which first took shape in the form of 

 physical laboratory experiments at Princeton and which are now be- 

 ing developed and extended in the laboratories of the Massachusetts 

 Institute of Technology. 



From every point of view it is advantageous for very high voltages 

 to have direct uniform currents. Van de Graaff was therefore led to 

 develop an electrostatic generator, since electrostatic methods yield 

 directly a steady unidirectional voltage such as is desired. Maximum 

 simplicity was sought in the design. The simplest terminal assembly 

 appeared to be a sphere mounted on an insulating column. Since the 

 sphere must be charged and since the process should be continuous, 

 the charge carrier should approach the sphere, enter it, and, after de- 

 positing its charge inside, should return parallel to its path of ap- 

 proach. This immediately suggested the action of a belt, a device 

 long used for the transmission of mechanical power. 



The logic of the situation therefore pointed directly to a generator 

 consisting of a hollow spherical conducting terminal supported on an 

 insulating column, a moving belt to carry electric charge to the 

 sphere, a device for depositing the charge onto the belt in a region of 

 low potential remote from the sphere, and a device for removing this 

 charge from the belt inside the sphere and transferring it to the 

 sphere. A refinement of these essentials was the addition of an in- 

 duction device whereby charge of the opposite sign was carried by 



