HIGH VOLTAGE — COMPTON 



257 



At the same time these positive charges on the metal sectors to 

 the right of G will similarly induce negative charges in the metal rod 

 AB^ which charges will be deposited on the metal sector to the left 

 of A and will in their turn be collected by the sharp point at E. 

 Thus the process is a continuous one, E and F collecting negative and 

 positive electricity, respectively, from the metal sectors on both of 

 the revolving disks. By having a multiplicity of revolving disks 

 these Wimshurst machines may be made to deliver a considerable 

 amount of power and were in fact 

 at one time quite largely used in 

 the X-ray art until they w^ere sup- 

 planted by the more powerful and 

 much more convenient electromag- 

 netic induction devices described 

 previously, including set-up trans- 

 formers, induction coils, and the 

 like. 



One of the most ingenious types 

 of electrostatic induction machines 

 is the famous Kelvin water-dropper 

 which is shown in figure 3. Here 

 perhaps more easily than in any 

 of the other induction machines 

 can be seen the way in which a 

 small charge once produced may 

 result in the continual building up 

 of an indefinitely large charge, if 

 the arrangement of apparatus and 

 coimections are suitably arranged. 

 Assume for a moment that for some 

 cause, such as friction of the wind 

 or anything else, there happens to 

 be a small charge on the cylinder -4. 

 Every drop of water leaving the outlet in A will therefore carry a 

 small induced negative charge which will be delivered to the cup 

 below, thus raising the cylinder at B to a negative charge. All of 

 the drops of water which come from the outlet inside of B will there- 

 fore carry the positive charges which will be collected in the trap 

 below and serve still further to increase the positive charge on A. 

 So the process goes on, the charges building up until through leakage 

 or through a corona discharge to the air, they leak away as fast as 

 ])roduced. 



At this point I am minded to make a confession regarding my 

 first experiment in physics, I conceived the idea of producing 



The Kelvin water-dropper. 



