54 Mr. E. H. Griffiths on the Influence of 



The above examples will show that it is possible to deter- 

 mine the resistance at 100 C C. to better than 1 part in 250,000, 

 and nearly the same order of accuracy is obtainable, with 

 proper precautions, in the vapour of sulphur. 



I believe the most difficult point to accurately determine is 

 freezing-point, and for really satisfactory determinations it is 

 advisable to use distilled-water ice ; but if this is manufactured 

 in the usual manner there is great danger that some trace of 

 salt may be carried from the freezing-mixture to the ice. 

 One precaution I have found advantageous when using 

 ordinary ice, viz., on adding water to the powdered ice it is 

 advisable to use water resulting from the melting of that ice 

 itself rather than either distilled or tap water. 



I will not burden this paper with any further detailed 

 account of the standardization of these thermometers. Between 

 March 4th and July 7th the fixed points were determined on 

 over 30 occasions, in ice, steam, or sulphur vapour, and no 

 variation which would affect temperature measurements be- 

 tween 0° and 100° C. by as much as 2000° ®* presented itself 

 during the latter half of these observations, the record and 

 reductions of which alone make a large pile of manuscript. 



The two thermometers, with their leads connected as 

 described, were placed at opposite ends of a bridge wire of 

 platinum-silver. During the spring of this year this wire was 

 subjected to a most careful calibration by what was practically 

 Carey Foster's method, and it proved to be more unequal than 

 I had expected. It was therefore re-calibrated by a different 

 method in which a resistance-box was used as a shunt, and the 

 agreement between the results was satisfactory. The whole 

 wire was 80 centim. long and had a total resistance of about 

 *4 ohm. For convenience, and to avoid thermal effects, a 

 similar wire connected with the galvanometer was laid along- 

 side it, and the sliding-piece was fitted with a screw so arranged 

 that a small turn of the screw-head made contact with both 



wires*, 



The wire and contact-maker were covered by a thick 

 copper shield (the screw-head projecting through a narrow 

 slit) passing from end to end of the bridge: thus the tempera- 

 ture of the wire was kept uniform. By means of a vernier the 

 divisions on the scale could be read to T ^ millim., which with 

 this wire and thermometers AB and CD indicated at 50° C. 

 a temperature difference of '00091 5° C.f The temperature- 



* This method of making the connexion is due to Professor Callendar, 

 and is exceedingly effective and convenient. 



t A thicker wire giving a more open scale was tried at first, but found 

 to be less convenient than the one finally adopted and calibrated. 



