164 Professor J. A. Fleming [May 21, 



current we describe as an alternating current, and if they execute 

 this motion very rapidly we call it an electric oscillation. 



The reason an electric current produces heat in a conductor is 

 because the drift energy of the electrons is then being continually 

 converted into additional irregular-motion energy in the free electrons 

 and atoms by collisions of electrons with the atoms of the conductor. 

 If, then, the temperature becomes very high, that is, if the irregular 

 electronic motion becomes very great, certain electrons may acquire 

 such velocities that they are flung out from the surface of the wire 

 even against the attraction of the positive atomic ions left behind. 

 If there is no electric force tending to make the electrons move away 

 from the neighbourhood of the hot wire these electrons constitute a 

 space charge around it, and the repulsion they exercise on each other 

 tends to keep other electrons from getting out into the space. 

 Suppose, however, that the incandescent wire is placed in the axis 

 of a highly exhausted glass tube, and is surrounded by a metal 

 cylinder which is kept positively electrified, the electrons move to it, 

 and others then make their exit from the wire. Such a tube with 

 incandescent wire cathode and cold metal plate anode is now 

 called a thermionic tube. The steady emission of electrons is 

 called a thermionic current. In the case of a tungsten wire brilliantly 

 incandescent in vacuo and under sufficient electric force, this current 

 may amount to as much as an ampere per square centimetre of 

 surface. This means that electrons are being flung or pulled out at 

 the rate of millions of billions per second per square centimetre. As 

 soon as Sir Joseph Thomson had proved by experiment that this 

 electronic emission was taking place, the explanation of the effects 

 observed in incandescent electric lamps by Edison, Preece and myself 

 became clear. For in the Edison experiment we have a slow drift of 

 electrons through the carbon filament superimposed on a very rapid 

 and erratic motion, and multitudes of these electrons are escaping 

 from the filament on all sides — just like steam escaping from a 

 porous or leaky canvas steam pipe. If the plate in the bulb is con- 

 nected to the positive pole of the filament-heating battery, it is 

 positively electrified and it attracts these escaped electrons, and they 

 enter it and drift through the external wire, forming the observed 

 Edison current. 



Suppose, then, that we connect the collecting plate by a wire 

 external to the bulb with the negative terminal of the filament, and 

 that we insert in this circuit a battery of a number of cells which 

 can be altered so as to vary the potential of the plate, the said 

 battery having its negative terminal connected to the filament, we 

 then find that a thermionic current flows which can be measured by 

 an amperemeter inserted in the circuit. If we vary the voltage 

 from zero upwards we shall find that the thermionic current increases, 

 but not indefinitely. It soon reaches a value at which no further 

 increase of voltage raises the current. If we plot our observations 



