702 SCIENCE PROGRESS 



action. This question forms a motif underlying the greater part of the book, 

 but the matter is specifically discussed in a section which is a model of minute 

 scientific analysis. Certainly in the case of tungsten, which, owing to its high 

 melting-point, is specially suitable for thermionic investigations, Prof. Richardson 

 has shown that no chemical action can occur ; but just lately {Phys. Rev., November 

 1916) Stoelkle has published an account of very careful experiments with molyb- 

 denum which show that Richardson's statement that " considerable changes in the 

 amount and nature of the gases present in the tube do not change the electron 

 emission " does not apply to that metal, though under suitable conditions it seems 

 that a purely thermal emission may take place. The commonly accepted equation 



for the variation of the saturation current with temperature, i = AT e~ r , can 

 be deduced equally well on each theory, and so far experiment cannot discriminate 

 between this equation and that obtained by the author (in which T' /2 is replaced 

 by 7*') both from the quantum theory, and from thermodynamics on the reason- 

 able assumption that the change of energy which accompanies the departure of 

 each electron from the hot body is a linear function of the temperature. This 

 matter seems to require more comment than it receives. Prof. Richardson says 

 "obviously both formulas cannot be correct"; but that both are equally well 

 satisfied by experiment does not seem very creditable to the experiments unless 

 the exact effect of the preponderating influence of the exponential term is 

 pointed out. 



The bearing of the work on contact potential difference is discussed at length, 

 and while this vexed question is still left open the recent experiments of Lester 

 seem to show that the excess of kinetic energy over the equilibrium value which 

 an electron, inside a metal, has to lose in order to escape with zero velocity, is the 

 same for many elements, from which it would follow that they should show no 

 contact E.M.F. in a good vacuum. In the chapter on the "Energetics of 

 Electron Emission," a full account is given of the experiments which the author 

 carried out, in collaboration with F. C. Brown, on the distribution of velocities 

 among the electrons — the first direct experimental verification of Maxwell's 

 distribution law. This chapter also includes an account of the latent thermal 

 effects. The measurements of the loss of energy which accompanies the escape 

 of electrons from metals is in good agreement with that indicated by the saturation 

 current equation ; but the experimental difficulties involved in the determination 

 of the gain of energy when an electron enters a metal have not yet been overcome 

 satisfactorily and only in the case of platinum is reasonable agreement obtained. 



The latter part of the book is devoted to the question of the emission of 

 positive ions from hot metals and of ions generally from heated salts and as a 

 result of chemical action. The thermodynamical and mathematical theory is not 

 obtrusive and can easily be avoided by those not specially interested in it. The 

 book can be recommended very cordially, not merely as an account of thermionics, 

 but for the instruction it gives in an experimental technique which requires a 

 knowledge of almost all branches of physics for its successful development. 



D. Orson Wood. 



A Treatise 011 the Theory of Alternating Currents. By Alexander Russell, 

 M.A., D.Sc, M.I.E.E. Vol. II. [Pp. xiii + 566, with 239 figures. Second 

 edition.] (Cambridge : at the University Press, 1916. Price 15^. net.] 

 The second volume of this well-known textbook is devoted to the theory of alter- 

 nating current machines and to questions pertaining thereto. It is intended for 



