164 Prof. Oliver Lodge on the Seat of the 
observation of the Volta effect for various difficult substances, 
especially liquids and liquids. 
Clifton arrives at the same conclusion with regard to sum- 
mation, and gives handy diagrams, reproduced in Jenkins’s 
‘ Hlectricity,’ of the contact-force at the different junctions. 
My own opinion is that the intended and obvious significance 
of these diagrams is theoretically wrong, but they embody 
certain experimental results conveniently, and they can be 
interpreted properly. 
Both Clifton and Ayrton and Perry appear to believe in 
the great constancy of the value Zn/Cu. Clifton gives it as 
8516 volt (“ Quelle précision ! ’’ somewhat sarcastically eja- 
culates Pellat, who himself finds it to vary between °63 and 
°92). Ayrton and Perry assert that it is more constant than 
a Daniell. I believe that both Professor Clifton and Pro- 
fessors Ayrton and Perry have made several experiments 
besides those recorded in their communications to the Royal 
Society ; but as they have not been published, I can give no 
account of them. 
Among the Théses presented to the Faculty of Science in 
Paris in 1881 we find an important memoir by Pellat*, 
which reviews the whole position very clearly, and records 
a series of determinations of Volta force among metals, 
determinations which are evidently the most accurate and 
satisfactory yet made. He adopts the capital experimental 
method of neutralizing the charge of a condenser bya Pog- 
gendortf or compensation method, and thus convefts Kohl- 
rausch’s into a null method, for which a very sensitive elec- 
troscope is all that is needed. The plates of the two metals 
set face to face are connected, not directly, but by a greater 
or less length of a graduated wire conveying a current ; and 
the position of the slider on the wire is adjusted by con- 
tinually separating the plates and testing until no charge at 
all is found. The step of potential on the wire is then pre- 
cisely equal to the “contact-force ”’ between the plates ; for 
this would have caused a charge in a similar but uncom- 
pensated condenser, and the step of potential on the wire has 
neutralized it. 
Compensation methods of a sort had been used before by 
* Theses présentées a la Faculté des Sciences de Paris, pour obtenir le 
Grade de Docteur-és-Sciences physiques, par M. H. Pellat, Professeur 
de Physique au Lycée Louis le Grand, No. 461; Juin 22, 1881. See 
also Journal de Physique, 1881, xvi. p. 68, and May 1 1880: ‘ Ditférence 
de potential des couches électriques qui recouvrent deux métaux en 
contact.” 
