196° ~ Prof. Oliver Lodge on the Seat of the 
In this connection I must notice also a rather long memoir* 
by Dr. W. von Zahn, published in 1882, which reviews the 
whole subject, and describes an elaborate series of measure- 
ments made with an apparatus something like what one might 
suppose Ayrton and Perry’s to become if it were arranged for 
use in different gases and in vacuo. He refers with admira- 
tion to Pellat’s work in the preface ; and I do not suppose 
imagines that his own numerical determinations can compare 
with Pellat’s for accuracy where they overlap, seeing that he 
only makes use of a sort of combination of Kohlrausch’s and 
Hankel’s methods, with a Hankel electrometer as a measuring 
instrument t. He has tried, however, a large number of sub- 
stances as well as ordinary metals, such as powdered antimony, 
iron and nickel reduced by hydrogen, many kinds of carbon, 
Fe,0,, manganite, pyrolusite, copper oxide, lead “ hyper- 
oxide,” iron glance, and other minerals. He has measured 
the Volta effect in various gases and at different pressures, 
and finds, like Pellat, that it does not appreciably vary. 
He has also examined the effect of temperature on the Volta 
effect, though he appears to think that this ought to bear some. 
close relation to the phenomenon of Seebeck, a natural mistake 
many years ago when made by Avenarius, whom it led most 
happily, though fortuitously, to the true, and by him experi- 
mentally verified, law of H.M.F. in a thermoelectric circuit f. 
However, Zahn finds that experiment lends no support to this 
view, and says that a larger series of results must be obtained 
before basing a theory on them. Von Zahn is a confirmed 
contact theorist, and he victoriously assails several experiments 
supposed to be distinctly in favour of a chemical view of the 
Volta effect. He says he publishes his results because of 
the extraordinary discoveries being propounded by Professor 
Hxner (such as that a thermopile will not work ina vacuum)§, 
* Untersuchungen tiber Contactelektricitét, von Dr. W. y. Zahn : Leipzig, 
Teubner, 1882. 
+ A Hankel electrometer is a modification of Bohnenberger’s, in which 
a battery with middle to earth is substituted for the dry pile; the plates 
on either side of the gold leaf are minutely adjustable, and the motions of 
the gold leaf are read by a microscope. It is sometimes preferred to a 
quadrant for its small capacity and dead quick motion; it can be made 
very sensitive, but it can hardly be a satisfactory measuring instrument. 
Pellat used it, but only as an electroscope. 
{ Avenarius, ‘4Die Thermoelektricitat, ihrem Ursprunge nach, als 
identisch mit der Contactelektricitat betrachtet,” Pogge. Ann. cxix. 1863. 
See also Pogg. Ann. cxxii., where he proceeds to calculate Volta effects 
from thermoelectric data. 
§ I have been unable to find this extraordinary statement in Exner’s 
works, but it is quoted again by Ayrton and Perry, Phil. Mag. 1881, p. 49. 
Exner seems to have said that the thermoelectric power of bismuth- 
