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XI. New Determination of the Mechanical Equivalent of Heat. 
By James Prescott Joule, D.C.L., LL.D., F.R.SS. L. and E., &c., President of the 
Literary and Philosophical Society of Manchester. 
Received November 15, 1877, — Read January 24, 1878. 
[Plate 26.] 
The Committee of the British Association on Standards of Electrical Resistance 
having judged it desirable that a fresh determination of the mechanical equivalent of 
heat should be made, by observing the thermal effects due to the transmission of 
electrical currents through resistances measured by the unit they had issued, I under- 
took experiments with that view, resulting in a larger figure (782 - 5)" than that which 
I had obtained from the friction of fluids (772 - 6).t 
The only way to account for this discrepancy was to admit the existence of error, 
either in my thermal experiments or in the unit of resistance. A committee, consisting 
of Sir Vm. Thomson, Professor P. G. Tait, Professor Clerk Maxwell, Professor B. 
Stewart, and myself, were appointed at the meeting of the British Association in 
1870 ; and with the funds thus placed at my disposal I was charged with the present 
investigation, for the purpose of giving greater accuracy to the results of the direct 
method. 
I’he plan I adopted was, in regard to the measurement of work, similar (as I after- 
wards found) to that used by Hirn, who has laboured so earnestly and successfully on 
this subject. Pie has described it as follows : — “ L’appareil qui m’a servi pour cette 
etude consiste : 1°, en un cylindre en laiton de 0 m '3 de diametre, de l m de longeur, 
poll a sa peripherie extern e, monte sur un axe solide en rapport avec un moteur d’un 
mouvement tres regulier, et pouvant recevoir une vitesse variant a volonte de 60 a 600‘ 
par minute ; 2°, en un cylindre fixe, poli a son interieur, concentrique an premier, 
eloigne partout de 0 m '03 de celui-ci. Les disques on plateaux formant les extremites de 
la cylindre etaient munis, a leur partie centrale, de boites a etoupes par oii sortait 
l’axe du cylindre interne. Tout 1 mtervalle compris entre les deux cylindres pouvait 
etre rempli ainsi d’un liquide quelconque que les boites a etoupes empechaient de 
s’ecotiler par les centres. 
“ Lorsque le cylindre interieur tournait, le frottement que sa surface externe 
exercait sur le liquide, et que le liquide, mis ainsi en mouvement lui-meme, exer^ait it 
* Brit. Assoc. Report, Dundee, 1867, p. 522. 
f Phil. Trans., 1850, p. 82. 
