134 DE. JOULE ON THE THEEMAL EITECIS OE C04IPEESSIXS PLUIDS, 
effect which actuaUy takes place at aU temperatures but that of ma^um or minimum 
The^ apparatus I employed consisted of a strong vessel 
4 inches in diameter. This vessel was connected at the upper ■ 
li inch internal diameter, furnished with a piston, by means of which the leq 
pCt could be laid on or taken off at pleasure. A 
Ld copper wires was placed in the centre of the copper vessel, the onfee through 
PP j . . tifrlit hv means of a plug of gutta percha. The outer jimction 
it was passed being made tight by means oi a pi g g i thprmo- 
was imLrsed in a bath of water, and the induced currents were measmed by the thermo 
multiplier in vacuo, described in my former paper. 
The method employed was alternately to lay on and remove weights from piston 
and"o TxamL the consequent deflection of the needle of the midtipher, fr-om which, b 
means of data derived from experiments made from time to time to *® “ 
of deflection arising from a change of temperature of the outer junction, 
p'fFppf sQiifflit' could, b© r©^dily d.©d.iic6d.* 
" fold that the needle took rather more than half a minute to assume a nev 
deflection. I therefore fixed upon 40" as the time allowed to ®1“P- 
tion or removal of pressure and the thermo-electnc observation. It was at first sus^ 
that the small cooling effect in consequence of the dilatation of the copper v 
internal pressure might interfere with the effect sought for; but it found on 
ttt a sumI application of heat to the outside of the copper vessel did not ^uslbll 
afect the temperltare of the central part of the liquid in which the jimction was piling ^ 
Itil In interval of time had elapsed equal to twice that occupied by a sivmg of the 
needle Tills source of error was therefore disregarded. 
Itolhe; possible source of error occurred to me. Was the thermo-electnc relation cl 
the metals employed to form the junction sensibly altered by the mtiuence o prebsu ^ . 
Thomson has lown that such an alteration in the metals copper and iron accompames 
the temporary strain produced by longitudinal tension*. dhe^^ir 
is however very minute, and, in the case of pressure uniformly applied m eieij toect 
:,!:rv:e are Lw considering, is probably far too smaU to be apprema ed by the m « 
delicate tests However, I made an attempt to ascertain whether it existed, by apply! ,, 
pressure when the temperatui-e of the outer junction was widely difterent fi-om tha t o 
thnrer the needle of the multiplier being kept in range by means of a contiolln g 
magnet ’ It was then found that the effect of pressure remamed the same as be oie. ai 
Thmefore the conclusion was necessarily arrived at, that the effect 
applied, in altering the thermo-electric relation of the metals, w-as, if it existed at all. 
* Pliilosopliical Transactions, 1856, p. ‘ H- 
