Assuming that the whole of the energy was converted into heat, when the air was subjected to a pressure of 21.5 atmospheres Joule obtained for the mechanical equivalent of heat about 824.8 foot-pounds, and when a pressure of only 10 .
P. Joule after years of patient labour in direct experimenting.
P. Joule with the perforated piston and with the friction of water and mercury.
By ca-sing two conical surfaces of cast-iron immersed in mercury and contained in an iron vessel to rub against one another when pressed together by a lever, Joule obtained 776.045 foot-pounds for the mechanical equivalent of heat when the heavy weights were used, and 774.93 foot-pounds with the small driving weights.
The size of jar commonly known as a quart size may have a capacity from 4 o 0 th to s 00 oth of a microfarad, and if charged to 20,000 volts stores up energy from a quarter to half a joule or from -ths to Iths of a foot-pound.
Now, we know that the number of electrochemical equivalents electrolysed is proportional to the whole amount of electricity which passed through the circuit, and the product of this by the electromotive force of the battery is the work done by the latter, so that in this case also Joule showed that the heat generated was proportional to the work done.
On account of its practical convenience, and its close relation to the international electrical units, the joule has been recommended by the British Association for adoption as the absolute unit of heat.
For a long time the final result deduced by Joule by these varied and careful investigations was accepted as the standard value of the mechanical equivalent of heat.
Bidwell's results for iron and nickel were confirmed, and it was further shown that the elongation of nickel-steel was very greatly diminished by tension; when 2 Joule believed that the volume was unchanged.
The deviations from the ideal volume may also be deduced by the method of Joule and Thomson.