576 Professor J. J. Thomson [April 19, 



particles forming the cathode rays only requires a mass of these 

 particles amounting to one ten-thousandth of a milligram, while to 

 carry the same charge by hydrogen atoms would require a mass of 

 one-tenth of a milligram.* 



Thus, to carry a given charge of electricity by hydrogen atoms 

 requires a mass a thousand times greater than to carry it by the 

 negatively electrified particles which constitute the cathode rays ; and 

 it is very significant that, while the mass of atoms required to carry 

 a given charge through a liquid electrolyte depends upon the kind of 

 atom — being, for example, eight times greater for oxygen than for 

 hydrogen atoms — the mass of cathode ray particles required to carry 

 a given charge is quite independent of the gas through which the 

 rays travel and of the nature of the electrode from which they start. 



The exceedingly small mass of these particles for a given charge 

 compared with that of the hydrogen atoms might be due either to the 

 mass of each of these particles being very small compared with that 

 of a hydrogen atom or else to the charge carried by each particle being 

 large compared with that carried by the atom of hydrogen. It is there- 

 fore essential that we should determine the electric charge carried by 

 one of these particles. The problem is as follows : Suppose in an 

 enclosed space we have a number of electrified particles each carrying 

 the same charge, it is required to find the charge on each particle. 

 It is easy by electrical methods to determine the total quantity of 

 electricity on the collection of particles, and, knowing this, we can find 

 the charge on each particle if we can count the number of particles. 

 To count these particles the first step is to make them visible. We 

 can do this by availing ourselves of a discovery made by C. T. R. 

 Wilson working in the Cavendish Laboratory. Wilson has shown 

 that, when positively and negatively electrified particles are present 

 in moist dust-free air, a cloud is produced when the air is closed by a 

 sudden expansion, though this amount of expansion would be quite 

 insufficient to produce condensation when no electrified particles are 

 present: the water condenses round the electrified particles, and, if 

 these are not too numerous, each particle becomes the nucleus of a 

 little drop of water. Now Sir George Stokes has shown how we can 

 calculate the rate at which a drop of water falls through air if we 

 know the size of the drop, and conversely we can determine the size 

 of the drop by measuring the rate at which it falls through the air ; 

 hence, by measuring the speed with which the cloud falls, we can 

 determine the volume of each little drop ; the whole volume of water 



* Professor Schuster in 1889 was the first to apply the method of the 



magnetic deflection of the discharge to get a determination of the value of — ; he 



e 

 found rather widely separated limiting values for this quantity, and came to the 

 conclusion that it was of the same order as in electrolytic solutions ; the result of 

 the method mentioned above, as well as those of Wiechert, Kaufmann and 

 Lenard, make it very much smaller. 



