September 30, 1921] 



SCIENCE 



287 



similar to that to which I have referred in 

 connection with thermionic emission, has ex- 

 isted for over a century. In 1Y92 Volta wrote : 

 " The metals . . . can by themselves, and of 

 their own proper virtue, excite and dislodge 

 the electric fluid from its state of rest." The 

 contrary position that the electrical manifes- 

 tations are inseparably connected with chem- 

 ical action was developed a few years later by 

 Fabroni. Since that time electrical investi- 

 gators have been fairly evenly divided between 

 these two opposing camps. Among the sup- 

 porters of the intrinsic or contact view of 

 the type of Volta we may recall Davy, Helm- 

 holtz, and Kelvin. On the other side we have 

 to place Maxwell, Lodge, and Ostwald. In 

 1862 we find Lord Kelvin * writing : 



For nearly two years I have felt quite sure that 

 the proper explanation of voltaic action in the com- 

 mon voltaic arrangement is very near Volta 's, 

 which fell into discredit because Volta or hia fol- 

 lowers neglected the principle of the conservation 

 of force. 



On the other hand, in 1896 we find Ostwald ^ 

 referring to Volta's views as the origin of the 

 most far-reaching error in electrochemistry, 

 which the greatest part of the scientific work 

 in that domain has been occupied in fighting 

 almost ever since. These are cited merely as 

 representative specimens of the opinions of the 

 protagonists. 



ISTow, there is a close connection between 

 thermionic emission and contact potential dif- 

 ference, and I believe that a study of ther- 

 mionic emission is going to settle this little 

 dispute. In fact, I rather think it has already 

 settled it, but before going into that matter 

 I would like to explain how it is that there 

 is a connection between thermionic emission 

 and contact potential difference, and what the 

 nature of that connection is. 



Imagine a vacuous enclosure, either imper- 

 vious to heat or maintained at a constant tem- 

 perature. Let the enclosure contain two dif- 

 ferent electron-emitting bodies, A and B. Let 



• Papers on Electrostatics and Magnetism, p. 

 318. 



° ' ' Elektrochemie, Ihre Geschiehte und Lehre, ''' 

 p. 65, Leipzig (1896). 



one of these, say A, have the power of emitting 

 electrons faster than the other, B. Since they 

 are each receiving as well as emitting elec- 

 trons, A will acquire a positive and B a nega- 

 tive charge under these circumstances. Owing 

 to these opposite charges A and B will now 

 attract each other, and useful work can be 

 obtained by letting them come in contact. 

 After the charges on A and B have been dis- 

 charged by bringing them in contact, let the 

 bodies be quickly separated and moved to their 

 original positions. This need involve no ex- 

 penditure of work, as the charges arising from 

 the electron emission will not have had time 

 to develop. After the charges have had time 

 to develop the bodies can again be permitted 

 to move together under their mutual attrac- 

 tion, and so the cycle can be continued an 

 indefinite number of times. In this way we 

 have succeeded in imagining a device which 

 will convert all the heat energy from a source 

 at a uniform temperature into useful work. 



'Now, the existence of such a device would 

 contravene the second law of thermodynamics. 

 We are therefore compelled either to deny the 

 principles of thermodynamics or to admit that 

 there is some fallacy as to the pretended facts 

 in the foregoing argument. We do not need 

 to hesitate between these alternatives, and we 

 need only look to see how the alleged behavior 

 of A and B will need to be modified in order 

 that no useful work may appear. There are 

 two alternatives. Either A and B necessarily 

 emit equal numbers (which may include the 

 particular value zero) of electrons at all tem- 

 peratures, or the charges which develop owing 

 to the unequal rate of emission are not dis- 

 charged, even to the slightest degree, when the 

 two bodies are placed in contact. 



The first alternative is definitely excluded 

 by the experimental evidence, so I shall pro- 

 ceed to interpret the second. It means that 

 bodies have natural states of electrification 

 whereby they become charged to definite poten- 

 tial differences whose magnitudes are inde- 

 pendent of their relative positions. There is 

 an intrinsic potential difference between A and 

 B which is the same, at a given temperature, 

 whether they are at a distance apart or in con- 



