232 BODIES SMALLER THAN ATOMS. 



the atom of hydrogen. This result is for the case when electricity 

 passes through a liquid electrolyte. I will now explain how Ave can 

 measure the mass of the carriers of electricity required to convey a 

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

 direct methods which are applicable to liquid electrolytes can not he 

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

 >olve 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 par- 

 ticles. If these are charged with electricity they ought to be deflected 

 when they are acted on by an electric force. It was some time, how- 

 ever, 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 

 tin 1 cathode rays make the gas through which they pass a conductor 

 of electricity; the particles are thus, as it were, moving inside conduct- 

 ing tubes which screen them off from an external 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 deflected by a magnet. The force 

 exerted on them by the magnetic held is at light angles to the magnetic 

 force; at right angles also to the velocity of the particle and equal to 

 II, v sin H % where // is the magnetic force, e the charge on the particle, 

 and 6 the angle between //"and y. Sir George Stokes showed long 

 ago that if the magnetic force 1 was at right angles to the velocity of 

 the particle the latter would describe a circle whose radius is mv < // 

 (if in is the mass of the particle); we can measure the radius of this 

 circle and thus find /// ve. To find v let an electric force F and a 

 magnetic force // act simultaneously 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 Ft just balances 

 that of the magnetic force which is equal to Hev; when this is the case 

 Fe—JSev or v—FH. AVe can thus And r, and knowing from the pre- 

 vious experiment the value of nn , . we deduce the value of //> e. The 

 value of in i' found in this way was about 10~ 7 ; and other methods 

 used b} r Wiechert, Kaufmann, and Lenard have given results not greatly 

 different. Since m/e=10~ 7 , we see that to carry unit charge of elec- 

 tricity by the particles forming the cathode rays only requires a mass 



