392 Messrs. Cross and Williams on the Strength of the 



metal were equalized, simply produce a current, but would 

 not itself be annulled. 



It would only disappear when tlie surface of contact was so 

 chemically altered as to preclude or fail to produce any elec- 

 trolytic action, in which case the new substance formed would 

 imply new conditions of surface-tension, and the hypothesis 

 that the alteration in surface-tension could be referred solely 

 to electrical forces on the double layer would be no longer 

 tenable. 



Most of these questions (depending, as they do, on a know- 

 ledge of the real differences of potential between metals and 

 electrolytes, which has not yet been obtained) would have been 

 set at rest if one could accept undoubtingly the experimental 

 results contained in the last part of MM. Exner and Tuma's 

 paper, which treats of the measurement of the contact- 

 difference of potential of metals and electrolytes. 



The potential of the electrolyte is tested by a mercury- 

 funnel drojjjnng within a pa[)er cylinder which is moistened 

 with the electrolyte in question, and in contact with the metal 

 which is put to earth. 



To get a correction for the zero of the instrument, the 

 funnel is afterwards made to drop inside a carbon or platinum 

 cylinder also to earth, which is assumed to give zero-})otential 

 at the funnel-point. It does so only if it may be assmned 

 that there is no contact-ditlerence between the platinum or 

 carbon and the air or water-film condensed on it; an assump- 

 tion which, though probably nearly correct, is as yet unproved, 

 and which therefore deti'acts frum the value of the results 

 obtained. 



XLVII. Tlie StrcDglli of the Induced Current iv'dh a Magneto- 

 Telephone Trans)ndter as Injivenced hy tlie Strength of tlie 

 Magnet. By Chakles R. Cross and Arthue S. Williams*. 



T is a well-known fact in practice, as well as an evident 

 consequence of theoretical considerations, that the 

 effectiveness of a magn(3to-telej)hone, when used either as a 

 transmitter or as a receiver, varies with the strength of the 

 magnetism of the core. But the relation of the one to the 

 other has never been studied, so far as we are aware. 



Our investigations include a study of the changes in 



* Commuuieated by the Autliors. Froui the Proceedings of the 

 Americau Academy of Arts and Sciences, vol. xxiv. (n. s. xvi.) p. 113. 



