32 



Mr. C. Chree. 



the junctions were exactly at the same depth and immediately below 

 the level of the platinum wire. The copper wires were tied with silk 

 to the vertical parts of the trestles, and, passing straight down, were 

 led through the bottom of the tub. The middle point of the platinum 

 wire was vertically below the centre of the dish, and thus its ends 

 were much nearer the axis of the tub than was the rim of the dish. 

 The strictly horizontal position of the wire was tested by pouring in 

 water first to the level of the wire and then to that of the dish. It 

 was thus made certain that when the tub was placed so that the dish 

 was horizontal, the platinum wire was so also. For most liquids the 

 pegs, wires, &c, were secured by sulphur, but for bisulphide of carbon 

 this was replaced by asbestos. During the experiments the tub was 

 placed in side a double-walled wooden box, the space between the walls 

 being stuffed with packing. The box was provided with a double lid 

 similarly stuffed. To the ends of the copper wires were attached 

 binding screws, the wires leading from which passed through grooves 

 cut in the rim of the box. Thus the wires were in no way disturbed 

 in moving the lid. This was a point of some importance, as even the 

 small variations in the electrical resistance produced by slightly dis- 

 turbing the binding screws was apt to affect the accuracy of the 

 observations. 



The platinum and its connecting wires formed one of the resistances 

 of a Wheatstone's bridge arrangement. One of the others was a fixed 

 resistance, and the remaining two were supplied by a wire bridge with 

 a sliding-piece. The resistance of the platinum wire varies with the 

 temperature, and — at least for small variations — its change is propor- 

 tional to the change of temperature. The current was supplied by a 

 single element — a small Daniell or Leclanche. A galvanometer, 

 whose resistance could be reduced to 012 of an ohm, measured the 

 variation from a balance between the resistances. The usual precau- 

 tions in dealing with small resistances had to be taken ; in particular 

 it was found difficult to avoid producing thermoelectric currents if 

 the sliding-piece were moved. 



In the first apparatus the tub was 38"2 cm. in diameter, and the 

 dish 30'2. The platinum was coiled in a spiral round a fine horizontal 

 glass tube at a mean depth of 6'45 era. below the liquid surface and 

 about 7 cm. above the bottom of the tub. The length of the spiral 

 was less than the radius of the tub. 



The method of conducting the experiment was as follows : — The 

 tub was put inside the box and filled with the liquid to the level of 

 the dish. The box having been adjusted till the dish was horizontal, 

 the lid was put on. The sliding-piece of the bridge wire was then 

 moved till no current traversed the galvanometer. As the tub and 

 liquid were in general at slightly different temperatures to begin with, 

 some time elapsed before the galvanometer reading became constant. 



