530 Dr. M. W. Travers and Mr. A. G. C. Gwyer. [Jan. 11, 



employ standard platinum, or did not succeed in maintaining sufficiently 

 steady temperatures, the results are not strictly comparable with those 

 already referred to. The results of Dewar and Fleming, and of Olszewski 

 have been summarised by Callendar ;* they show varying divergences 

 from the parabolic formula, depending on the kind of platinum 

 employed. Holborn and Wienf compared the platinum, air, and 

 hydrogen scales at temperatures between 300 and 500, and again at 



- 78 and at - 190. They employed a platinum coil enclosed within 

 the gas thermometer bulb, so as to minimise the error due to varying 

 temperature. They found with platinum for which a was equal to. 

 0-00378 the parabolic formula gave a very close approximation between 



- 78 and 500, but showed a deviation of 2 '3 at - 190. We find 

 that the deviation becomes marked even at - 78. 



Apparatus Employed. The thermometer consisted of a coil of 

 platinum wire about 130 cm. long, having a resistance of 7 ohms at 

 the ice point. The ends of this wire were fused to leads about 0*3 mm. 

 in diameter, and these leads and the ends of the compensating leads 

 were soldered respectively to the ends of four insulated copper wires 

 enclosed in a lead tube. The resistance was measured by means of 

 resistances constructed by the Cambridge Scientific Instrument Co., to 

 the design of Professor Callendar. The individual resistance coils were 

 carefully calibrated by one of us before the experiments were 

 commenced. As the coils were of platinum-silver alloy, for which the 

 temperature coefficient is 0*00024, it was necessary to observe their 

 temperature each time an observation was made, and to reduce the 

 result to zero. The resistance plugs were cleaned before each experi- 

 ment, and the zero point on the sliding resistance, which rarely varied 

 by more than 0*0001 ohm between individual sets of experiments, or 

 by more than 0*0005 ohm in the course of two years, was always 

 observed. A home-made galvanometer of the two-coil astatic type was 

 employed, and with the highest resistance in the circuit it was possible 

 to adjust the slider with an accuracy corresponding to 0*0002 ohm. 

 In connection with these instruments the usual arrangement of keys, 

 etc., was employed. 



The Fundamental Interval of the Thermometer. In determining the ice 

 and steam point the platinum coil and leads were enclosed in a glass 

 tube containing some paraffin to the height of about 3 cm. above the 

 coil. An identical result for the ice point was obtained by protecting 

 the thermometer by means of a glass tube with holes in the side of it 

 and immersing it directly in melting ice and water. The following are 

 the results of the experiments, which need not be given in detail : 



* Phil. Mag.,' February, 1899. 



t ' Wied. Ann.,' 1896, 59, p. 213; 1901, 6, p. 242. 



