2S4: 



Mr. P. E. Shaw : 



plete, the atomic agitation is reduced, but coherence is now 

 accomplished. 



When the wires are pulled asunder the bridge is snapped ; 

 but traces of orientation may be seen for some time when 

 the surfaces are brought together again, as shown in Tables VI.. 

 VII., & VIII. 



The difference between the slate in the wire and at the 

 contact will be evident from the simple relation 



n = r.q, 



where n = no. of ions per c.c; T = mean free time; q — no. of 

 ions formed per sec. per c.c. 



Now the velocity of ions might vary as the potential gradient 



— , which would be enormous in the bridge ; but the 



mean free-path would increase probably at the same time, so 

 that t would not increase fast enough to prevent n and q from 

 becoming very great. 



It is possible that some part is played by ionization of the 

 air-film between the wires ; but, so far, there seems no direct 

 evidence of this. 



9. Results of other Investigators. 



Daring the last three years a great deal of experimental 

 work on the nature of coherence has been published. The 

 theories of Lodge* and Branly f, the pioneers of the subject, 

 are well known ; these opposing views have been subjected 

 to many and varied tests. The view of Lodge has been 

 repeatedly shown to stand the test of experiment, and is at 

 present pre-eminent. The following brief summary will give 

 some idea of the experimental results so far obtained : — 



Aschkinass % considers that, since chemically clean copper 

 coheres well, the oxide film is not essential to coherence 

 phenomena. [But a condensed air-film may exist apart from 

 any oxide as such ; his observation makes the case for a 

 condensed air-film stronger.] 



Blondel § found that the thickness of the film of oxide or 

 sulphide on the metal filings must not be greater or less than 

 a certain amount. [I shall show later that a very thick film 

 will not be bridged over when discharge occurs ; also a thin 

 film will not provide sufficient insulation.] 



m not proviue sumcieni msuiauuii.j 



* Phil. Mag. 1894, xxxvii. p. 94; Electrician, 1897, 

 f Comptes Rendus, 111. p. 785, and 125. p. 939. 

 % Wiedemann's Ann. 1898. 

 § Eclair aye Electrique, 1898. 



p. 87. 



