104 



MR F. E. SMITH ON THE CONSTRUCTION OF 



was therefore higher than the ice-point. The results of the observations are as 

 follows, the temperature recorded being that of the mercury in contact with the 

 thermometer bulb. 



TABLE X. Erection of Standards by Method III. 

 Resistance Measurements by Carey Foster Bridge. 



For reference, the values of the resistances when the standards were erected by 

 Method II. are given in column 4 of the above table ; differences are tabulated in 

 column 5. If the end corrections and temperatures were identical in the two cases, 

 there should, of course, be no difference between the values (a) and (/>). The two 

 measurements of P, when the conditions of temperature were different, indicate that 

 the observed differences are largely due to a temperature effect. Indeed, the 

 conduction of heat through the leads (they were 0'6 centim. in diameter) forbids the 

 attainment of a steady known temperature. When the leads were removed from the 

 platinum thimbles, and the sheaths plugged with cotton wool, the mercury was found 

 to fall in temperature to the ice-point. If, when the mercury was at this temperature, 

 the leads were quickly reintroduced, and a measurement made, a slightly lower value 

 for the standard resulted. Such an observation was, however, very unsatisfactory, 

 and, in order to obtain more definite information, the following experiments were 

 made. 



The tube V was chosen for the observations. It was erected by Method III. as 

 before, and left in the ice box for 12 hours previous to any measurement. No copper 

 leads were, however, introduced, the sheaths being plugged with cotton wool. The 

 thermometer, also, was removed, a platinum thimble being substituted for it. Into 

 the four thimbles, thin platinum wires passed ; the outer two of these were intended 

 for battery leads, the inner two as potential points. The resistance of the 



