316 Notices respecting New Boohs. 



0. W. Richardson, on the escape of ions from incandescent 

 solids, which he finds to agree with the requirements of the 

 gas-law, supports the former view, in fact seems to be its 

 main experimental support ; he concludes, moreover, (Phil. 

 Trans. 1906) that the number of free electrons in platinum 

 ranges somewhere about 10 21 , which is not discordant, under 

 the circumstances, with the general estimate J. 10 23 made above. 



The conclusion reached above, that in fact the free electron 

 is deviated or entangled by each molecule which it meets or 

 traverses, also points to the smal'er velocity, as the rapid 

 electrons are known to pass straight -through thin sheets of 

 metal. But the free path now seems hardly long or definite 

 enough to substantiate, except very roughly, Drude's formula 

 for metallic conduction as effected entirely by the steady 

 deflexion of the free paths by the electric field. If the rough 

 general estimates o£ order of magnitude, made above, for the 

 number of free electrons and their velocities and free paths 

 in the best conducting metals, are substituted in Drnde's 

 formula [loc. c/7.), they give a conductivity of the order 

 12. 10 15 , which is about one-fiftieth of that of silver, and errs 

 on the wrong side, though hardly so far as to cause surprise 

 when the necessary vagueness of the data is kept in view. 



Cambridge, July 16. 



XXVII. Notices respecting New Books. 



Exercices et Projets cV ElectrotecJinique. Publies sous la direction de 

 Eric Gerard et Omer De Bast, Tome Premier. Paris : 

 G-authier-Villars. 1907. Pp. 240. 

 r FHIS is the first instalment of a collection of problems primarily 

 -*- intended for the use of students attending the electrotechnical 

 courses at the Moutefiore Iustitute. It is devoted mainly to 

 general magnetic and electrical theory, and deals with magnetic 

 moments, intensity of magnetization, electrostatic theory, the laws 

 of the electric current, electromagnetism, electromagnetic induction, 

 and alternate current theory. This latter subject is treated very 

 fully, and with special reference to its technical applications. The 

 total number of problems is 101, aud each problem is followed by 

 a fully-worked solution. A book of this type should prove of 

 great use to all students of electroteehnology, but especially so to 

 those who are more or less thrown on their own resources, and 

 can only occasionally take advantage of: the guidance of a teacher. 

 The solutions supplied should have an excellent effect in training- 

 students to those systematic methods of attacking problems which 

 so many find it difficult to acquire. Altogether, the work is to be 

 very highly commended, and should find a wide sphere of usefulness 

 as a companion volume to Eric Gerard's well-known Lecons sur 

 TEJectricite. 



