34 



Mr. C. Chree. 



small cell a deflection exceeding 300 divisions of a millimetre 

 scale could be obtained for a rise of one degree in the tempera- 

 ture of the platinum. When so sensitive as this, however, the 

 galvanometer was too much exposed to the disturbing influences 

 of adjacent currents or the movement of magnets in neighbouring 

 rooms. From eighty to a hundred scale divisions to a degree usually 

 gave the best results, and had the advantage of keeping the spot of 

 light near the centre of the scale during the whole experiment, with- 

 out any movement of the sliding-piece. The sensitiveness was most 

 easily determined by finding how far the sliding-piece on the bridge 

 had to be moved, when the battery was on, to produce a given change 

 in the galvanometer reading. This test was usually applied at the 

 beginning and end of each experiment, as a change in the sensitive- 

 ness during the observations might lead to erroneous conclusions. A 

 slight displacement of the controlling magnet of the galvanometer 

 may occur without affecting to any noticeable extent the position of 

 the zero, and so without some such test as the above a chauge in the 

 sensitiveness might escape detection. 



Theory. 



Let v denote the temperature, p the density, c the specific heat, and 

 h the conductivity of a given liquid. Suppose the liquid to extend 

 to infinity in every direction, and over the entire plane x — a 

 uniform supply of heat to be distributed at a rate given at the time t, 

 counted from the first application of the heat, by the function f{t) 

 per unit area ; then at the distance x from the plane of application 

 the temperature at time t is given by 



In the present experiment the base of the dish answers to the plane 

 x — 0, and f(t) is to be regarded as proportional to the rate at which 

 heat is conveyed from the . dish to the liquid. It is true of course 

 that the liquid exists only on one side of the plane x = 0, and does 

 not extend to infinity in any direction. Doubtless the base of the tub 

 tends to reflect the heat that has passed downwards through the 

 liquid, but in the apparatus actually used any such reflected heat 

 would be extremely small, and only the most trifling part of even this 

 effect would show itself within the time of the experiment. Since 

 the length of the platinum wire was much less than the diameter of 

 the dish, which was in turn considerably less than that of the tub, 

 the limitation in the horizontal direction would appear of small conse- 

 quence. In fact when a larger quantity of hot water than was usually 

 employed was poured into the dish, a delicate thermometer indicated 



