On the Lags of Thermometers. 49 



3. The partial separation achieved was proved by density 

 measurements. The density difference found between the 

 heaviest and lightest mercury amounts to 0*49 % , corre- 

 sponding to a difference of 0*1 unit in the element weight of 

 mercury. 



In carrying out the experiments described here we are 

 indebted to Mr. V. Thal-Jantzen for his valuable assistance. 



Physico- Chemical Laboratory of the 



Polytechnic Institute of Copenhagen. 



IV. On the Lags of Thermometers. By A. R. McLeod, 

 M.A,, Fellow of Gonville and Cuius College^ Cambridge *. 



IN a former paper f expressions have been given for the 

 steady lags in the mean temperatures oE thermometers 

 with spherical and cylindrical bulbs, when the external 

 temperature increased at a constant rate and the initial tem- 

 perature was zero everywhere. Under the same conditions, 

 Dr. T. J. I'A. Bromwich % has since given formulae for the 

 mean lags at all times until the steady value is reached. 

 The present paper deals among other things with the lags 

 for such thermometers, when the initial and surface con- 

 ditions are somewhat different. The conditions were 

 suggested to the author by Mr. E. Gold, and are based on 

 experience in taking air temperatures by aeroplane in 

 France. 



The new conditions are obtained by warming up the 

 thermometer initially to a fixed temperature, the external 

 temperature being thereby initially unchanged, and by 

 having a discontinuous surface condition, the temperature 

 of the air (or other medium) changing linearly with time 

 until a certain time £ 1? and then changing linearly at a new 

 rate. 



Numerical values for mercury and alcohol agree with 

 those given in the former paper. Using the same thermo- 

 meter stem with each bulb, the alcohol cylinder is better 

 than the mercury cylinder and both are decidedly superior to 

 spheres. Owing to the fact that much finer capillary tubes 

 may be used with mercury than with alcohol, the possibility 

 exists of constructing mercury thermometers with much less 



* Communicated by the Author. 



t Phil. Mag. xxx vii. Jan. 1919, p. 134. 



% Phil. Mag. April 1919, p. 407. 



Phil. Mag. S. 6. Vol. 43. No. 253. Jan. 1922. E 



