Researches in Absolute Mercurial Therrnometry. 



87 



probable error of the separate results for the various constants is abou.t 



0-oor c. 



The Constant of CtqnIIari/ Depresmn K and the Coefficient of Extermil 

 Pressure. — It is usual to determine the pressure coefficient by suspending 

 the thermometer in a tube, the pressure within which can be rapidly 

 changed from atmospheric pressure to one of a few centimetres of 

 mercury. A sudden diminution of pressure j) causes a fall in the 

 indications of the thermometer, and in the absence of capillary effects 

 8:p would measure the so-called coefficient of external pressure. But 

 the fall of thermometer being accompanied by a change in the shape of 

 the meniscus, the readings before and after the change of pressure are 

 not directly comparable. If the meniscus is normal at high pressure 

 (which can be realised in the experiment, by arranging for a slowly 

 rising temperature), we must add to the reading at the low pressure a 

 certain constant K, which represents the difference in the readings of a 

 thermometer between a rising and falling thread. Hence (S -i- K)//.? Avill 

 be the corrected coefficient of external pressure ). If ten sets of 

 observations are made, with different changes of pressure {pi, po) giving 

 different falls of the thermometer (S^, S.,), we may put 



,^ _ ^1 + K _ ^2 + K 



and hence K ^^l^izM , ^ . 



In the actual observations the changes of temperature which take 

 place between the readings must, of course, be allowed for, and the 

 equations only hold if these changes are so slow that at the low 

 pressure the actual temperature has not overtaken the apparent tem- 

 perature at the moment the reading is taken. Previous observers not 

 being interested directly in the quantity K, have arranged their experi- 

 ment so that the readings of low pressure were only taken after a 

 time sufficient to allow the rising temperature to have its effects, so 

 that the thread was rising in all observations. Mr. Sworn, on the 

 other hand, wishing to determine K and /3e simultaneously, had to 

 arrange the experiment so that at low pressures the hydrostatic 

 pressure in the bulb was the same as with a falling thread. 



The following table gives an idea of the consistency of the results 

 obtained : — 



