THE MRCHANICAL EQUIVALENT OF HEAT. 
373 
Section II. —The Caloeimetee and Coil. (See Plate 2, fig. 2.) 
The calorimeter, which was of a cylindrical form, was made of “ gilding metal ”— 
73 per cent, copper and 27 per cent. zinc. 
The weight of the lower part was 501'84 grms. ; of the lid 245'67 grins. 
The internal diameter and the depth were each 8 centims. 
Its capacity was therefore about 400 cub. centims. 
The weight of the calorimeter may appear excessive for this capacity. However, 
as any “panting” of the lid or base would have thrown the stirrer out of adjustment 
and as the pressure on the flat base of the calorimeter would amount to about 
120 lbs., when the space surrounding it was vacuous, it was evident that considerable 
thickness was necessary to secure rigidity. The two parts of the calorimeter were 
held together by eight screw bolts ; a lead ring previously placed between the parts 
was crushed by the tightening of the screws and thus the join was rendered air¬ 
tight. It was therefore necessary to make both the lid and the projecting lip of the 
calorimeter sufficiently strong to bear the strain without bending. 
As we expected to determine the water equivalent with as much accuracy as other 
quantities, the large mass of the calorimeter did not appear to be a serious objection. 
The whole of the calorimeter was heavily gilded both inside and out, and the 
surfaces polished. The lead ring''' was covered witlr gold leaf because considerable 
quantities of PbCOs were formed if the water had access to it; thus, no metal 
except gold was exposed to the action of the water. An advantage of the gold 
exterior was that its surface did not tarnish, and thus its coefficient of radiation 
remained constant. 
Fixed on the interior of the lid were eight metal tubes, 1 centim. in length. Three 
of these were 18 millims. in diameter and served to hold the glass tubes by which 
the calorimeter was suspended. The others were 5 millims. in diameter and supported 
the glass rods on which the coil was wound. These rods reached to within a couple 
of millimeters of the base when the lid was placed in position. Two copper rods, 
7'5 centims. in length (14 B.W.G.) passed from the insulated junctions at M and N 
(fig. 2, p. 382) to nearly the bottom of the calorimeter. These were also heavily 
gilded, and the ends of the platinum wires were fastened to their lower extremities. 
The last coil used by us was about 13 inches in length, and was wound round the 
lower end of the glass rods in such a manner, that the difference of potential of any 
two adjacent points could never be great. Thus the first 6 inches were wound on one 
side, the remainder on the further side of the calorimeter. A depth of 1'5 centim. 
of water completely covered the whole coil. 
The glass tubes passing through the lid were held in position by perforated india- 
rubber corks. Great pressure had to be exerted to force the tubes “ home,” and 
* A pure gold ring was tried, but it was found impossible to crush it sufficiently to render the join 
air-tight. 
