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



investigation, viz. the thermometry. The method I proposed 

 to adopt necessitated extremely accurate measurements of 

 small differences of temperature. The actual elevation was 

 of little consequence, and therefore the use of differential 

 thermometers immediately suggested itself. Two platinum 

 thermometers (labelled AB and CD) were constructed with 

 great care: four stout platinum leads passed down the stem of 

 each, supported and insulated in the usual manner by small 

 disks of mica, and the resistance of all these leads was made 

 as equal as possible before attaching the coils. Great atten- 

 tion was given to this matter, and it is safe to assume that the 

 leads in no case differed amongst themselves by 1 in 10,000. 

 The coils, consisting of a particularly pure sample of platinum 

 wire, were then attached, and several days were devoted to 

 securing their equality. Their resistance in ice was about 

 18 ohms : thus ysqq of their resistance could be directly 

 determined in the box. The galvanometer swing was about 

 500 for a change of "01 in the box ; and such equality was 

 secured that when both thermometers were placed in ice (the 

 necessary precautions being taken with regard to exterior 

 connexions &c), no difference in the swing of the galvano- 

 meter could be observed ; thus they differed by a quantity 

 certainly less than 1 in 100,000. This equality, although 

 not a necessity, was a great convenience. 



Although cut from the same length of wire and insulated 

 in a precisely similar manner, the coils did not possess exactly 

 the same coefficients. The resistances in steam and sulphur 

 were repeatedly determined and checked by observations in 

 the vapour of aniline. Both thermometers were on several 

 occasions heated to a red heat, the hard glass tubes containing 

 them becoming slightly bent in the process ; but since this 

 annealing no further change has been observable in them. 

 The method of completely standardizing such instruments 

 has been fully described by Professor Callendar and myself 

 in Phil. Trans. 1891, A, and I need not therefore here dwell 

 upon it. The values of 8 differed slightly, viz. P513 and 

 1*511 ; but such a difference, even if not allowed for, would 

 over the range 0° to 100° 0. in no case cause an error 

 exceeding about 2^00° C. in elevation. These thermometers 

 were so connected that the compensating leads of A were 

 placed in series with the coil of B, and vice versa. Any 

 heating of the stem of A or B, therefore, added an equal 

 resistance to each arm of the bridge ; and as the leads w T ere 

 everywhere bound together, the indications were absolutely 

 independent of all changes in temperature except those of the 

 bulbs. 



