the Heat-conductivity of Stone. 248 



arrangement of such a nature as would not add to or subtract 

 from the heat at the centre. 



One thermoelectric junction beiug at the centre of the ball, 

 the other may be either kept at a constant temperature, in 

 which case the electric current will be a function of the dif- 

 ference of temperature between the junctions, their mean 

 temperature, the position of the neutral point for the two 

 metals employed, and the slope of their thermoelectric lines ; 

 or the other junction may be immersed in a compensating-bath, 

 which, by being always kept at such a temperature that there 

 is no current, indicates at any moment the temperature of the 

 centre of the ball. 



This latter, or balance, method was adopted, as it has the 

 following advantages: first, the range of temperature through 

 which the ball falls may be large, and still the galvanometer 

 may be made as sensitive as we please ; second, the balance 

 method may be employed nearly without reference to the 

 " thermoelectric power " of the two metals at different tem- 

 peratures ; and, thirdly, since there is no current, no heat is 

 added to or subtracted from the centre of the ball thermo- 

 electrically, so that no vitiations of the theoretical conditions 

 to ensure accuracy could occur except by heat-conductivity 

 of the thin wires. In reality, of course, as it was impossible 

 to cool the compensating-bath at exactly the same rate as the 

 centre of the ball, there usually was a very weak current ; but 

 as the junction in the bath was as often a very little hotter as 

 a very little colder than that in the ball, the excessively small 

 gains and losses of heat produced by the currents balanced 

 one another. 



However, our balance method was employed chiefly for the 

 first two reasons, and not because we feared the abstraction 

 of heat through thermo-electric currents. 



II. Details of the Apparatus. — In PI. IX. fig. 1, A is a stone 

 ball 13*8 (thirteen and eight tenths) centims. in diameter, rest- 

 ing on three points in a metal water-bath, B, 17*5 (seventeen 

 and a half) centims. high and 18*3 (eighteen and three tenths) 

 centims. in diameter. This bath had a tap, R, for letting in 

 cold water of constant temperature from a cistern, and a large 

 opening, 0, which could be closed by a cork, for suddenly 

 emptying B. The bath stood in a tub, W, to catch the over- 

 flow, as will be described further on. At the centre of the 

 stone ball there was a thermoelectric junction, C, of iron and 

 copper, the wires being carefully insulated from the water and 

 from one another except just at their extremities, where they 

 were bound together and soldered, and immersed in a small 

 drop of mercury to form good thermal contact with the stone. 



R2 



