324 POPULAR SCIENCE MONTHLY. 



too, by the results of experiments on electrolysis, that to carry the 

 unit charge of electricity requires a collection of atoms of hydrogen 

 Avhich together weigh about ^/jq of a milligram; hence if we can 

 measure the charge of electricity on an atom of hydrogen we see that 

 ^/lo of this charge will be the weight in milligrams of the atom of 

 hydrogen. This result is for the case when electricity passes through 

 a liquid electrolyte. I will now explain how we 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 appli- 

 cable 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 produce the well-known green phosphorescence on 

 the glass of the tube. These rays are now known to consist of nega- 

 tively electrified particles moving with great rapidity. Let us see how 

 we can determine the electric charge carried bv a given mass of these 

 particles. We can do this by measuring the effect of electric and mag- 

 netic 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 suc- 

 cess 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 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 field is at right angles to 

 the magnetic force, at right angles also to the velocity of the particle 

 and equal to Hev sin where H is the magnetic force, e the charge 

 on the particle and 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 mv/eH (if m is the mass of the particle) ; we can measure the 

 radius of this circle and thus find m/ve. To find v let an electric force 

 F and a magnetic force H act simultaneously on the particle, the elec- 

 tric 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 Hev; when this 

 is the case Fe = Hev or t; = F/H. We can thus find v, and knowing 



