10 l)i i ' ' i . . 'ions on 



TaMe II. Resistance of Compensator Loops. 



Thermometer K, K i\ K, K K K 7 



istance (in ohms) 0'20 0'22 0-15 <H5 0-21 O'l'O U i"-' 



We may thus infer that the average resistance of the connecting wires 

 is some 1 per cent, of that of the spiral in one of the ordinary thermo- 

 meters. Thus a small differential change between these wires and the 

 compensator loop might appreciably affect the thermometer readings. 



The Cambridge Instrument Company inform me that the connecting 

 wires and the compensator loop which are of the same thickness of 

 wire are of the same platinum as the spiral and have the same 

 temperature coefficient. If this were not the case a small difference 

 between the absolute resistances of the connecting wires and the com- 

 pensator would introduce a source of error fluctuating with the 

 temperature of the wires and compensator. 



It would, I think, be desirable to make sure that the process of 

 attaching the thick wires to the fine spiral causes no change in the 

 material close to the junction. 



Chinges in the Leads or in the Proportional Arms, 



9. In the leads supplied to Kew Observatory one pair has the 

 terminals at both ends marked P, the other pair has its terminals 

 marked C. This is intended to show that the former pair should be 

 attached to the terminals marked P on the thermometer head and to 

 the plugs P in the box, and that the other pair should be attached to 

 the C terminals on the thermometer head and the plugs marked C in 

 the box. Supposing this arrangement invariably adhered to, any 

 'ititftrnt difference between the resistances of the leads should not 

 appreciably influence the apparent resistance of a platinum ther- 

 mometer. Supposing, however, that a difference between the resist- 

 ances of the leads develops itself, then if the leads are used in the 

 way indicated above, the effect on the readings is the same as if the 

 platinum thermometer itself changed. 



Loosening and tightening the leads had been found early in the 

 investigations to be a possible cause of slight variations in the reading 

 owing presumably to the varying tightness and it may allow the 

 terminals to blacken during the sulphur point experiments. It was 

 thus at first thought undesirable to touch the lead connections once 

 the thermometer was attached, especially as it was supposed that if 

 error were at all likely to arise in so obvious a way, provision would 

 have been made for detecting and eliminating it. Thus the matter 

 did not receive consideration until 1897, when the erratic nature of 

 some of the results drew attention to the fact that the leads were 

 becoming worn near the ends. 



