Platinum Thermometry at Kew Observatory. 



33 



been accountable for the fact that the mean values of the melting 

 point of silver from experiments by Mr. Hugo and myself (six in all) 

 were only 958'2 C. with K x and 957"'5 with K 2 ; while Mr. Heycock 

 and Mr. Neville, at Cambridge, found 961'l C. with the same sample 

 of silver but a different thermometer. This is 2 

 individual observation at Kew Observatory. 



above the highest 



Slowness of Platinum Thermometers in Acquiring Hie True 

 Temperature. 



30. When a mercury thermometer is taken from a room at one 

 constant temperature into another room at a different constant tempera- 

 ture, some time elapses before the reading becomes steady ; while, if 

 the temperature of the surrounding medium alters, the thermometer 

 shows a lag which varies with the thickness of the glass walls and the 

 amount of mercury in the bulb. 



As we have seen, the naked coils in the Kew resistance box have a 

 smaller lag than an ordinary mercury thermometer, but it is otherwise 

 with platinum thermometers. Here the wire does not come into direct 

 contact with the external medium, and the attainment of the steady 

 state is somewhat slow. 



Experiments have been made with some of the Kew thermometers 

 which have been suddenly transferred from a bath about 15 C. into 

 ice, or into a hypsometer in which the water is freely boiling. The 

 interval, in seconds, was observed which elapsed before the bridge-wire 

 reading came to 1, 0'5, or other fixed distance of the final stationary 

 position ; this position had been found in a preliminary experiment. 



In all cases the immersion was similar to that of the ordinary fixed 

 point experiments. 



Two mercury thermometers, Nos. 686 and 750, were experimented on 

 in an exactly similar way for the sake of comparison. They are both 

 Kew standards of the following dimensions : 



I have not thought it necessary to go into full details of the observa- 

 tions. The following table gives the number of seconds required to 

 get to 1 and - 5 of the stationary temperature in ice, and to 1, 0'5, 

 and 0'25 of the stationary temperature in steam. It was found that 



VOL. LXVII. D 



