446 Professor 0. W. Richardson [May 7, 



an incandescent lamp, using one in which the filaments are not 

 anchored, we see that the loops are attracted by a negatively charged 

 rod, Init not by one which is charged positively. They show, in fact, 

 a behaviour which is precisely opposite to that of a wire at a dull red 

 heat in air. 



Xow let us consider the nature of the ions which carry these 

 thermionic currents, to use a term which I have ventured to apply to 

 the currents which leak away from the surfaces of hot bodies in this 

 manner. As is well known, the negative electrons which play such 

 an important part in physical phenomena are very readily deflected 

 by moderate magnetic fields, whereas ions of atomic or greater 

 magnitude are not. I have here an arrangement which will enable 

 us to apply this test to the ions emitted by hot bodies. An exh lusted 

 tube carrying a horizontal hot wire is placed in a vertical electric 

 field. The electric field is arranged so as to drag the negative ions 

 emitted by the wire to a suitable electrode, whence they flow through 

 a galvanometer whose deflection is registered by the spot on the 

 screen. Around the tube an electro-magnet is arranged, so that, 

 when it is excited, there is a horizontal magnetic field which tends 

 to curl up the paths of the ions. If I now switch on the electro- 

 magnet, you observe that the current is at once reduced to a small 

 value, showing that the magnetic field curls up the paths of the ions, 

 so that they are now unable to reach the electrode. The carriers of 

 this negative discharge are, in fact, electrons. 



I have here a second tube, arranged to give a conveniently large 

 positive discharge. When this is tested by the electro-magnet in a 

 similar way, the magnetic field is found to have no influence on the 

 thermionic current. The positive ions are, in fact, much more massive 

 than the electrons ; more elaborate experiments have shown that they 

 are charged atoms. 



We see from these experiments that the negative emission is 

 characterized by the electronic nature of the carriers and by its 

 permanence in a vacuum. The presence of a gaseous atmosphere 

 is not necessary in order to maintain these currents. Thus the 

 electrons must come from the heated body itself. I believe that 

 this emission is a process which is closely analogous to evaporation. 

 The essence of evaporation, of a liquid for example, lies in this : 

 that, as the temperature is raised, the molecules acquire sufficient 

 energy to overcome the forces which attract them to the liquid, and 

 so bi^come free molecules of the vapour. We know that all material 

 subs':ances contain electrons, and it is not unreasonable to expect 

 them to behave, when the temperature is high enough, in a way 

 analogous to the molecules of a liquid. Another analogy, in some 

 ways more accurate, would liken the emission of electrons to the 

 reversible evolution of a gas by the decomposition of a solid such 

 as Cilciam carbonate. The similarity of this process to evaporation 

 is well known to chemists. 



