18 



< hree.' Investigations on 

 Table VL 



sulphur point observations with a particular thermometer, the KO, KI, 

 and R* would be wrong to the same amount, so that the fundamental 

 interval and pt, (the value of pt answering to the boiling point, of 

 sulphur) would be unaffected. In practice, however, we cannot antici- 

 pate so favourable a contingency. 



After some experiments, I found that I could at pleasure change the 

 amount, and even alter the sign of the r-d difference by heating with 

 the finger or cooling by an air-blast one of the terminals of the 

 Griffiths key. 



Heating due to the Battery Current. 



17. In the Kew apparatus there is an option of two resistances, 

 viz., 100 and 20 ohms, in the battery circuit ; and our original instruc- 

 tions were to use the 100 ohms when observing with an ordinary 

 thermometer in ice and steam, and the 20 ohms when observing at the 

 sulphur point. The object, doubtless^was to ensure sufficient galvano- 

 meter sensitiveness at high temperatures. 



In an early sulphur experiment I was surprised to find that the 

 reading was about 0> 07 higher with the 20 ohms in use than with the 

 100. There being only a single dry cell in use, I had not anticipated a 

 sensible difference. Supposing the cell-to [remain constant, the heating 

 effect would of course remain the same for a given thermometer at any 

 one fixed point, and so would not influence the results immediately in 

 view. It seemed, however, inexpedient to trust to this, and we have 

 accordingly employed the 100 ohm resistance in all the regular observa- 



