166 Prof. Oliver Lodge on the Seat of the 
Gerland and by Thomson. Grerland applied compensation to 
determine the .M.F. of two metals dipped into a liquid, and 
Thomson applied it to the divided-ring experiment, thus making 
it very analogous to that of Pellat *. 
Pellat also adopts Sir William Thomson’s view that the 
Volta effect is due to a true contact-force between the metals, 
and that it represents a real difference of potential between 
them when in contact ; at the same time he is careful to point 
out that no rigorous proof can be given of this, and that all 
that is really and certainly measured electrostatically is the 
difference of potential between what he calls the electric coats 
(les couches électriques), or what may be more simply called 
the air-films, on the two touching metals. 
The following is a summary of some of Pellat’s measure- 
ments, gold being the metal with which all are compared, and 
the numbers being given in volts. 
‘¢ Difference of Potential of the Hlectric Coats which cover a 
Metal and Standard Gold metallically connected, and both 
Woy Jeanie," 
H.M.F. in volts. E.M.F. in volts. 
With surface 
| strongly 
With very | With surface | With very scratched 
Metals used.| clean but strongly || Metals used.{ clean but | by rubbing 
scarcely scratched scarcely with emery, 
seratched | by rubbing scratched jor in some of 
surface. with emery. surface. the last cases 
with cloth or 
filter-paper. 
ZAR CPR Sees "85 1:08 Ne MarOTaeeice cae "29 “38 
ead ses ‘70 crue Br aSSycosce ‘29 ey 
4 DLA eesti ere ‘60 ‘73 || Copper ...... 14 "22 
Antimony ... “44 “45 Platinum ... —'03 +:°06 
Nickel v5.0: "38 "AD Golda. aie: — “04 +07 
Bismuth...... “36 "48 Silvera. ease: —°06 +04 
Steel... 2... "29 “44 
* I find indeed that Sir W. Thomson completely anticipated Pellat in 
the application of galvanic compensation to the measurement of Volta 
effects; for in ‘Nature,’ April 14, 1881 (vol. xxiii. p. 567), is printed an 
account, given in brief at the Swansea meeting of the British Association 
(see Trans. of Sects. 1880, p. 494), which relates how the divided-ring 
experiment naturally developed into more complete compensation with 
slide resistances, and that an extensive series of measurements were made 
on this plan in the years 1859-61 with results quite in agreement with 
those published by Hankel in 1862. Other experiments were made since 
1861 with results confirmatory of those of Pfaff, 1821, showing the Volta 
effect to be independent of the surrounding gas. The description of all 
these experiments was therefore withheld till something new should be 
obtained by the method, and was not published until Pellat’s paper had 
