1901.] on the Existence of Bodies Smaller than Atoms. 575 



given charge of electricity through a rarefied gas. In this case the 

 direct methods which are applicable to liquid electrolytes cannot be 

 used ; but there are other, if more indirect, methods by which we can 

 solve the problem. The first case of conduction of electricity through 

 gases we shall consider is that of the so-called cathode rays — those 

 streamers from the negative electrode in a vacuum tube which pro- 

 duce the well-known green phosphorescence on the glass of the tube. 

 These rays are now known to consist of negatively electrified particles 

 moving with great rapidity. Let us see how we can determine the 

 electric charge carried by a given mass of these particles. We can 

 do this by measuring the effect of electric and magnetic forces on 

 the particles. If these are charged with electricity they ought to be 

 deflected when they are acted on by an electric force. It was some 

 time, however, before such a deflection was observed, and many 

 attempts to obtain this deflection were unsuccessful. The want of 

 success was due to the fact that the rapidly moving electrified particles 

 which constitute the cathode rays make the gas through which they 

 pass a conductor of electricity ; the particles are thus, as it were, 

 moving inside conducting tubes which screen them off from an ex- 

 ternal electric field ; by reducing the pressure of the gas inside the 

 tube to such an extent that there was very little gas left to conduct, 

 I was able to get rid of this screening effect and obtain the deflection 

 of the rays by an electrostatic field. The cathode rays are also de- 

 flected by a magnet ; the force exerted on them by the magnetic field 

 is at right angles to the magnetic force, at right angles also to the 

 velocity of the particle, and equal to He» sin 0, where H is the 

 magnetic force, e the charge on the particle and 6 the angle between 

 H and v. Sir George Stokes showed long ago that, if the magnetic 

 force was at right angles to the velocity of the particle, the latter 



would describe a circle whose radius is -= (if m is the mass of the 



particle) ; we can measure the radius of this circle, and thus find — . 



v e 

 To find v, let an electric force F and a magnetic force H act simul- 

 taneously on the particle, the electric and magnetic forces being both 

 at right angles to the path of the particle and also at right angles to 

 each other. Let us adjust these forces so that the effect of the 

 electric force which is equal to Fe just balances that of the magnetic 



force which is equal to ~B.ev. When this is the case Fe = H.ev, or v = 

 ■pi 



— . We can thus find t, and, knowing from the previous experiment 



the value of — , we deduce the value of — . The value of — found in 

 e e e 



this way was about 10 -7 , and other methods used by Wiechert, Kauf- 



mann and Lenard have given results not greatly different. Since 



- = 10 -7 , we see that to carry unit charge of electricity by the 



