Platinum Thermometry at Kew Observatory. 25 



value, t g = 444 '53, then in the Kew thermometers, as will be seen 

 later, 8=1'5 nearly, and we have approximately 



At the ice point A = bpt x 0'985 = 0'066 (T - 20), *| 



steam A/ = Apt x 1-015 = 0'094 (T - 20), I ... (9). 

 sulphur & = Ap^x 1-134 = 0-200 (T- 20) J 



23. Supposing that we aim at an accuracy of - 001 C., and are 

 able to keep our coils within 1 C. of the standard temperature,, then it 

 will suffice to know our temperature coefficient correctly to 1 part in 

 66, to 1 part in 94, or to 1 part in 200, according as the " fixed point " 

 concerned is the ice, steam, or sulphur point. 



Accuracy to 0'001 C. at the boiling point is not a very extravagant 

 aim, and it thus appears desirable to know the temperature coefficient 

 correctly within 1 per cent, unless we can keep nearer than 1 to the 

 standard temperature. From correspondence with Mr. Griffiths, I 

 found that he did not anticipate so high a degree of accuracy as this in 

 his determination of the temperature coefficient, and as a matter of fact 

 two fairly complete experiments by Dr. Marker have given values from 

 0'00025 to 0'000245 for combinations of several of the actual box 

 coils. 



The allowance of a departure of 1 C. from the standard temperature 

 is a narrow one, except under specially favourable conditions. A good 

 thermo-regulator, no doubt, can control the gas supply with consider- 

 able accuracy, even under unfavourable conditions; but what the 

 thermo-regulator controls is its own temperature. That temperature is 

 probably in general close to the mean temperature of the immediately 

 surrounding water. But supposing, as may happen in the platinum- 

 thermometer room at Kew, that the air temperature is only 14 one 

 day and 19 the next, then the temperature of the coil chamber will be 

 much higher on the second day than on the first, supposing the thermo- 

 regulator to have been left untouched. One cannot tell in advance 

 what is going to happen during the night, and if one finds in the morn- 

 ing the box thermometer indicating 17 and attempts to bring it up to 

 20", then either a long time is wasted in waiting during a very gradual 

 rise, or else one is very uncertain as to the uniformity of the tempera- 

 ture in the coil chamber. 



Again, in summer the room may be at 22 or 23, or more, and the 

 only simple way of keeping the coil chamber at 20 viz., constant 

 supplies of cold water in the bath would probably introduce uncer- 

 tainties greater than those it removes. 



On the whole, experience at the Observatory suggests that it is best 

 to keep the box nearly at the temperature of the room, so that seasonal* 

 variations of T between 17'5 and 22 '5 have been common. Supposing 



* In the course of a single daj's experiments the temperature changes were usually 

 only a few tenths of a degree. 



