488 



REPOET — 1884. 



them. 1 He considers the electrical effect brought about by the electro- 

 negative ion oxygen combining with the zinc and charging it negatively, 

 -while some electro-positive ion combines with the copper and charges it 

 positively, ' though not unless the two metals are in sufficient proximity to 



overcome electrolytic diffusion ' [whatever 

 that may mean as applied to this case]. 

 He thinks his mercury dropping experi- 

 ment in coal gas is conclusive as to the 

 equality of potential of metals in contact. 

 This, I fear, is rather rapid induction. I do 

 not see how it follows on his own hypothesis 

 that his arrangement is virtually a reversed 

 Lippman electrometer. 



Sir W. Thomson's dropping arrangements 

 or voltaic cells, in which gravity does the 

 work instead of chemical action, are so well 

 known that it is scarcely necessary to do 

 more than refer to them. Mr. Hart's mer- 

 cury dropper is scarcely a modification of 

 the copper-filing dropper shown in fig. 12. 

 Sir William also shows how to couple up 

 such cells in series, 2 and how to construct 

 a mechanical replenisher on the Volta 

 d copper funnel. rinciple (fig. 13). 

 The copper filings drop negatively g j n Qrder to iye th j g historical sketch 

 charged against electrical forces. If . ° ., , ,, 



you join c and d by a copper wire more completeness, it may be as well to- 

 you can get a current flowing record rapidly such other memoirs as I 

 wholly through and with copper. baye been able to get acquainted with : it 



is in the highest degree probable that several are omitted, but I hope 

 no very important ones. Professor Wiedemann's collection of views and 



Fig. 12. 



—Thomson's Gravitation 

 Voltaic Cell. 



a copper filings. 

 b inductor — zinc. 



<* receiver. 



FlS. 13. — Thomson's Voltaic Induction Machine. 



One of the inductors t is lined with one metal, the other with another, and the two connected. 

 The carrier wheel is rotated, and the contact springs a a' become oppositely charged. By 

 afterwards charging the inductors with a Oaniell cell, and comparing the deflection now 

 produced in an electrometer connected lo A a' with what it was before, measurements of 

 Volta effect can be obtained ; or of course it can be made a null method. 



memoirs bearing on the subject is at the end of the second volume of the 

 new edition of his ' Elektricitiit.' 



Edlund has published a long paper 3 in which he investigates experi- 

 mentally the Peltier effect ; he points out clearly at the end that there is 



1 The diagrams of potential which Mr. Hart gives of cells were given more fully 

 by Prof. Exner in his paper on the Theory of Galvanic Elements, 1880. Mr. Hart's 

 views are, in fact, rather similar to si our of the more reasonable ones of Prof. Exner. 



2 Electrostatic Reprints, p. 32.>. 



3 Edlund: Por/ff. Ann. exxxvii. 471; cxl. 435 ; cxliii. 401, 534. See also F&il. 

 May. (4) xxxviii. 263 ; xliii. 81, 213, 2G1 ; especially p. 273. 



