THE FORMS OF ENERGY. 121 



account of his earliest experiment on the mechanical equivalent (Scientific 

 Papers, i. p. 123). In this he measured the work done in turning a small 

 machine, which we should now call a dynamo, and he also measured the 

 current produced. From his law of heating effect due to current, discovered 

 shortly before, he was able to determine the total heat evolved in the 

 circuit of the dynamo from a measurement of that evolved by a known 

 current in a known resistance, and hence he found the mechanical 

 equivalent to be 836 foot-lbs. per pound of water heated 1 F. In a 

 postscript to the paper describing this experiment he says that by 

 working a piston perforated by small holes, forming narrow tubes, in a 

 cylindrical glass jar holding about 7 Ib. of water, each Ib. of water was 

 heated 1 F. by the work equivalent to raising 770 Ib. 1 foot. In the 

 following year he gave as the results of experiments on the work done in 

 the compression of air and the heat generated, 823 (Scientific Papers, i. p. 

 171), and in a continuation of this work he was led to devise the experi- 

 ment on the expansion of air described above. The result of this con- 

 tinuation was 798. In 1845 he first described a method in which 

 falling weights were employed to churn water in a calorimeter, and the 

 mechanical energy lost by the weights was taken as equivalent to the 

 heat developed in the water. The result was 890. He then proceeded 

 to improve the conditions of this last and most direct experiment, and in 

 1850 a full account was published in the Philosophical Transactions 

 (Scientific Papers, i. 298). The general nature of the apparatus will be 

 seen from Fig. 75. Two masses, each either 10 Ibs. or 29 Ibs., were 

 attached by strings each to the axle of a wheel and axle wa, iva. From 

 the wheel strings passed horizontally to the drum d, attached to a spindle 

 on which were fixed paddles of brass, 8 in number, revolving in a calori- 

 meter C of copper and containing a known weight, about 7 or 8 Ibs. of 

 water. The drum d could be detached from the spindle so that the 

 masses could be wound up to a height of 5 feet from the floor without 

 rotating the paddles. When they were wound up the drum was re- 

 attached to the spindle, and as the masses fell the paddles spun round. 

 In the calorimeter were fixed four brass vanes, cut out like the wards of 

 a lock. These allowed the paddles to pass through them, but prevented 

 any continuous circulation of the water and therefore any permanent 

 acquisition of kinetic energy by it. The water was only churned up by 

 the motion of the paddles, and its kinetic energy was rapidly transformed 

 to heat through fluid friction. The arrangement inside the calorimeter 

 is shown by the horizontal and vertical sections in the figure. The 

 masses had a terminal velocity on reaching the floor varying from 1'4 to 

 3'1 inches per second. This was noted and allowed for as, in effect, 

 diminishing the height of fall. The wheels were mounted on bearings 

 with as little friction as possible, but the residual friction was calculated 

 by subsidiary experiments, and the amount of kinetic energy absorbed 

 by it was allowed for. The calorimeter was on a wooden stand with 

 transverse slits, in order that the calorimeter should rest on a few points 

 of the wood only. Loss by conduction was thus reduced to a very small 

 quantity. 



In each experiment the masses were wound up and allowed to fall 

 twenty times, the duration of an experiment being somewhat over half an 

 hour. The rise of temperature of the water in the calorimeter was then 



