450 Professor 0. W. Richardson [May 7, 



each other, and are maintained at the same potential. Yerticallv 

 above this lower plate and a short distance away from it is a parallel 

 metal plate connected to the insulated quadrants of an electrometer. 

 An arrangement is provided by which a suitable difference of potential 

 can be maintained between the two plates, so as to oppose the motion 

 of tbe electrons from the strip towards the upper plate. It is clear 

 that if the electrons have no velocity when they are emitted, any 

 retarding field, however small, will be sufficient to stop them from 

 reaching the upper plate and charging up tbe electrometer. If, on 

 the other hand, they are shot off with a definite component of 

 velocity normal to t he strip, they will reach the upper plate, provided 

 the corresponding kinetic energy exceeds the work they have to do to 

 overcome the opposing difference of potential. Thus, if the electrons 

 are not at rest when they are emitted, they will give rise to currents 

 capable of flowing against an applied electromotive force if this is not 

 too large. I have here an arrangement, similar in principle to that 

 just described, which will enable me to show to you the existence of 

 these currents flowing against an applied electromotive force. The 

 platinum strip is replaced by a very short tungsten filament, the 

 upper plate by a surrounding cylinder, and the electrometer l)y a 

 galvanometer. The apparatus is thus different in detail from that 

 already referred to, but the principle is the same. You observe that 

 the current is largest when the opposing difference of potential is 

 zero, and falls off uniformly and rapidly as the potential difference 

 is increased. By increasing the temperature I can cause a consider- 

 able current to flow- against an opposing difference of potential of 

 one volt. 



The experiments just referred to are a kind of electrical analogue 

 of the high jump, in which the measuring tape is replaced ])y a 

 voltmeter. Corresponding to each emission velocity there is a definite 

 equivalent voltage. The fact that the current falls off continuously 

 as the opposing voltage increases shows that the electrons are 

 not emitted with a single velocity, but with different velocities 

 extending over wide limits. Careful experiments of this kind have 

 enabled us to discover what proportion of them are shot off with 

 velocities within any stated limits, to determine, in fact, what is the 

 Law of Distribution of Velocity among the emitted electrons. 



More than fifty years ago Maxwell concluded, from rathf r abstruse 

 theoretical considerations, that the velocities of the molecules of a 

 gas or vapour should not all be equal, but should be distributed in a 

 certain way about the average value. This law, known as ]\Iaxweirs 

 law^ of distribution of velocity, is somewhat similar to that which 

 governs tbe density of bullet-maiks on a target at different distances 

 from the bull's-eye. The theoretical considerations which led Maxwell 

 to establish this law for gases apply equally to the atmospheres of 

 electrons outside hot bodies. Let us see whether the results of our 

 experiments agree with Maxwell's predictions or not. If the law^ of 



