Platinum Thermometry at Kevi Observatory. 27 



tin calibrated mercury thermometer, even Avith an open scale, cannot 

 be relied on to give temperature differences to this degree of accuracy 

 under the most favourable conditions. 



The thermometer in use in the Kew box is a good calibrated one, 

 and errors in its graduation are unlikely to exceed 0'01. It is, how- 

 ever, subdivided only to fifths of a degree, and individual readings 

 cannot claim an accuracy of 0'01. 



Fortunately, errors in reading merely introduce irregularities in the 

 results, and so are not of fundamental importance in the present 

 inquiry. 



A. more subtle source of trouble is the secular change of zero, normal 

 to mercury thermometers. A rise of 0'l in the zero which is not 

 very much in excess of what actually occurred during the four years 

 occupied by the experiments would, in fact, exactly simulate a rise of 

 0-007 in the value of K . 



In the ordinary use of platinum thermometers, at least when there 

 are moderately frequent observations of RO, the circumstances are 

 different. The slow secular change of zero in the mercury thermo- 

 meter is then of little moment, while great importance attaches to the 

 accuracy of the individual readings. 



Failure of the Box Mercury Thermometer to give the True Temperature of the 

 Coils and Bridge Wire. 



. 25. If the room temperature is below 20 C., and artificial heating 

 is employed, we may have a practically stationary reading on the box 

 thermometer, and yet find it differing by several tenths .of a degree 

 from a second thermometer whose bulb is at a different level in the 

 coil chamber. Under such circumstances the temperature inside the 

 coil chamber also varies with the distance from the side or end 

 walls. 



The coils are of various shapes and diameters, some coming much 

 nearer the top and bottom of the coil chamber than others, and they 

 are necessarily at different distances from the ends. Thus, under the 

 conditions specified above, different coils may possess different mean 

 temperatures, and the temperature given by the mercury thermometer 

 may differ sensibly from that of any one of the coils. Even with no 

 artificial heating of the box, the temperatures at different parts of the 

 coil chamber may differ by several tenths of a degree, when the room 

 temperature is changing moderately fast. In such a case the presence 

 of water in the protecting tank seems a positive drawback, as it makes 

 the temperature of the coil chamber lag behind that of the rest of 

 the room. 



When the temperature of the coil chamber is altering, whether 

 through artificial heating or otherwise, the various coils doubtless alter 



