in Terms of the Mechanical Equivalent of Heat. 3 



meter. The stirrer consisted of a spiral blade of sheet copper 

 supported on a brass frame, the upper part of which was 

 tubular, and passed through the central tube of the cover. 

 The stirrer was kept in motion during the experiment by a 

 silk thread, which passed over a vulcanite wheel at the top of 

 the stirrer and ran to a driving-clock. The stirrer formed 

 the escapement of the clock, which ran very uniformly with 

 this arrangement. I estimated the heat generated by the 

 stirrer as two thirds of the whole work of the weights. This 

 is about one thousandth part of the heat generated by the 

 current, and only a rough determination of the correction is 

 needed. The thermometer passed through the tubular upper 

 part of the stirrer, and was clamped to a shelf above in such 

 a manner that its bulb was in the centre of the calorimeter 

 and surrounded by the stirring-blade, which, in turn, was 

 surrounded by the wire which carried the current. The wire 

 was composed of an alloy of platinum and iridium, and was 

 varnished to prevent conduction to the water. Its resistance 

 was about 1*8 ohm. The calorimeter was supported on legs 

 of vulcanite within a copper vessel with double walls, the 

 space between which was filled with water. This water-jacket 

 was provided with a hollow cover, also filled with water, and 

 its inner surface and the outer surface of the calorimeter were 

 nickel-plated and polished. Thus the calorimeter was nearly 

 surrounded by an envelope of fairly constant temperature, 

 the thermometer, stirrer- thread, and connecting wires passing 

 through openings in the jacket. 



From the mercury-cups, in which the electrodes of the 

 calorimeter dipped, the wires of the main circuit ran to the 

 battery and galvanometer. These wires were 2*5 millim. in 

 diameter, cotton-covered, carefully paraffined, and twisted 

 together to eliminate direct action on the needle. The battery 

 consisted usually of 24 one-gallon bichromate cells, arranged 

 4 in series and 6 abreast, and gave a very steady current. In 

 one experiment only 20 cells were used, 4 in series and 5 

 abreast. The galvanometer-coil for the main current was a 

 single turn of stout wire laid in a groove on a wooden circle 

 of about 80 centim. diameter. A sine-galvanometer was so 

 placed that its needle was in the axis of the single-wire coil 

 and about 1 centim. distant from its plane. This excentricity 

 was rendered necessary by the length of the suspending fibre. 

 The coil of the sine galvanometer was connected with the 

 calorimeter- electrodes by a second circuit, in which a resist- 

 ance-coil of 30,000 ohms was included. The wires of this 

 circuit were kept apart, as the current was too small to exert 

 an appreciable direct action, and as great irregularity in some 



B2 



