8 Messrs. C. W. Waidner and F. Mallory on 



exposed to the same temperature changes. These compen- 

 sating leads are adjusted so that their resistance is equal to 

 that of the main leads P P. Now, obviously, if these leads 

 C C are placed in the other arm of the Wheatstone-bridge, 

 as shown in fig. 2 below, they will exactly compensate for 

 the change in resistance of the leads to the coil, and conse- 

 quently the resistance we measure, and hence the temperature, 

 is independent of the depth of the immersion of the ther- 

 mometer in the liquid or vapour whose temperature is sought. 

 A little consideration will show that it is not necessary that 

 the resistance of the compensating leads be exactly equal to 

 the leads to the coil in order to have compensation. It is 

 only necessary that they be so nearly equal that the tempera- 

 ture changes of their difference produce no appreciable effect 

 on the resistance. 



Calorimeter. — The calorimeter, in which these comparisons 

 were made (shown in fig. 1), was made of copper and 

 provided with an efficient stirrer, which was driven by a 

 small hot-air engine. The outside of the calorimeter was 

 closely wrapped with a coil of single silk-covered german- 

 silver wire, so that the temperature of the calorimeter 

 and its contents could be raised at any desired rate by an 

 electric current of suitable strength. The calorimeter was 

 placed inside a large double-wall copper vessel (ext. diam. 

 36 cm., ht. 35 cm.), which surrounded it on all sides except 

 the top. The space between these walls (2*5 cm. apart) could 

 be filled with water at any desired temperature, and thus the 

 temperature of the calorimeter could be maintained constant 

 for some minutes or caused to vary at will. The entire 

 arrangement was then surrounded by a large box (46 cm. on the 

 side), and all intervening spaces packed loosely with feathers. 



Resistance-Box. — The resistance-measurements of the plati- 

 num thermometer were made with a Callendar- Griffiths 

 resistance-box (No. 7) especially designed for the measure- 

 ment of platinum temperature. The construction and cali- 

 bration of a resistance-box similar to the one used by us has 

 been described by Mr. E. H. Griffiths in ' Nature/ Nov. 14th, 

 1895. A general idea of the construction of the box may be 

 gathered from the following diagrammatic sketch. 



A l5 A 2 are two equal coils which serve as the equal arms 

 of a Wheatstone-bridge. The coils of the box, which were 

 made of platinum-silver, have approximately the following 

 nominal values in box-units ( approximately, one box unit = 

 0-01 ohm) :— 



HGFEDC B AFI 



5 10 20 40 80 160 320 640 100 



