Conduction of Heat in Liquids. 



33 



When this had occurred the sliding-piece was again moved till there 

 was no current through the galvanometer. It was then unnecessary 

 to move the sliding-piece again, unless the deflection became greater 

 than was usual in the experiment. When the galvanometer reading 

 had remained some time constant, the lid of the box was removed and 

 some hot water rapidly poured into the dish, care being taken that 

 none splashed over into the tub. Sometimes the lid was immediately 

 replaced and left on during the whole of the experiment ; on other 

 occasions the water was after a certain interval removed by a siphon, 

 ready filled for the purpose. This always left a small quantity of 

 water sufficient to cover the base of the dish without separating into 

 drops. 



The battery was connected with a key, and there was another in the 

 galvanometer circuit. In the earlier experiments these were depressed 

 in close succession at intervals of one minute, and the consequent 

 kick or deflection of the needle observed. ' Subsequently it was found 

 more convenient to use a constant battery, and to keep both keys 

 down during the whole course of the experiment and for some time 

 previously. Both methods were employed for most of the liquids 

 examined, and no difference was detected in the results. The tem- 

 perature of the platinum wire was seldom raised as much as 2° C. 

 during the experiment, and consequently the disturbance of the 

 balance in the Wheatstone's bridge was small. Thus the current 

 through the galvanometer could be taken as directly proportional to 

 the change in the resistance of the platinum, and so to the rise in its 

 temperature. 



Immediately subsequent to the application of the hot water there 

 was a decided increase of the galvanometer reading which ceased 

 very shortly. The reading then remained almost stationary for 

 several minutes. It then began to increase rapidly and continued to 

 rise for a considerable time, though the rate of change began com- 

 paratively soon to decrease. At first with the larger tub it was 

 attempted to determine the interval that elapsed before the reading 

 ceased to increase. This was, however, found impracticable, as it 

 required several hours to reach this epoch ; and after an hour and a 

 half the rate of change was so slow that the least variation in the 

 temperature of the laboratory was sufficient to upset the experiment. 

 Even with the smaller apparatus this was not a quantity to be con- 

 veniently observed. It was found much easier to determine the 

 much shorter interval that elapsed before the platinum wire was 

 being most rapidly heated. This interval could also be expressed 

 conveniently by means of the mathematical theory in terms of the 

 conductivity and other properties of the liquid, and so its determina- 

 tion was sufficient for the purpose in view. 



The galvanometer could be made so sensitive that with a single 



VOL. XL1II. D 



