THE MECHANICAL EQUIVALENT OF HEAT. 401 
British Association), had a resistance of 2‘5co; and the third, of copper, had a 
resistance '5(0* 
Fig. 6. 
The arms AD, DC (about 6 feet each) were made of uncovered copper wire. A 
preliminary rough adjustment could be made at D and the final adjustment of the 
whole bridge was made by altering the temperature of the coil in the arm AB. This 
alteration in temperature was produced by the heat developed in the wire itself, and 
thus the whole calorimeter was under the same conditions as those prevailing during 
our determination of J. 
The temperature on thermometer E,,; was read at the moment when the galvano¬ 
meter (Gj) showed no deflection. The behaviour of the galvanometer indicated that 
the flow of heat from the coil was not quite steady, but took place in gushes, owing 
probably to the coating of insulating varnish on the wire ; for when the key was 
depressed, the mirror did not swing steadily back to its zero, but oscillated about it, 
and the moment had to be estimated when the mean of these oscillations coincided 
with the zero. That this could be done with considerable accuracy is showui by the 
fact that the individual observations agreed well amongst themselves. 
The general conduct of a series of these experiments was as follows. 
We first found the temperature of the coil when the bridge was in adjustment, the 
E.M.F. at the ends of the coil being that used when taking a resistance in the 
ordinary way. When the calorimeter was cooled down, the E.M.F. was raised and 
the coil allowed to warm up until a new balance was obtained and the temperature at 
that moment noted. 
As the temperature and consequently the resistance of the arm BC remains constant, 
and since the arms AD, DC are practically equal, their ratio remains unchanged. 
The resistance of the arm AB (the coll) must also be the same as before, hence the 
difference in the two observed temperatures gives us the number of degrees that the 
coil is hotter than the surrounding water; or we can express the result in terms of 
increase in resistance, as the resistance (whenE = *004) at all temperatures is known. 
* The total mass of metal in this arm of the bridge amounted to several pounds. 
MDCCCXCIII.—A. 3 F 
