204 



harper: thermometric lag 



a suitable basis for the development of methods of determining X 

 under a given set of conditions, and for computing the lag cor- 

 rections in cases of applied thermometry where the corrections 

 are appreciable. 



Variation of X with stirring, and the lag of a thermometer as a 

 fluid current meter. — The lag of the same thermometer in water, 

 viscous kerosene, and air, each forced past the bulb at different 

 velocities covering a wide range, furnished the data from which 

 is taken the table below. The method employed was to plunge 

 a previously cooled thermometer into a fountain of the fluid, gush- 

 ing out of one arm of a vertical U-tube, the other arm of this tube 

 being connected to a source of supply maintained at constant 

 temperature. A suitable throttle gave the desired current, which 

 was measured by weighing the spill in a given time, or by a gas 

 meter. The velocity of flow past the bulb was obtained from 

 the current and area of the annular orifice. 



TABLE 1 

 X in Seconds. Small Bulb "Chemical" Thermometer 



"Infinite stirring" means bringing the wall of the bulb instantane- 

 ously to its final temperature and maintaining it there by an "infinite" 

 heat source while the average temperature of the thermometer changes 

 from its initial to its final value. The X corresponding to this condition 

 should be a real constant of the thermometer, bearing no relation to the 

 medium of immersion. The condition is probably very closely realized 

 for a small thermometer when it is plunged into live steam, in view of 

 the high latent heat of the steam, and the value 2.2 seconds was so 

 obtained. 



The agreement between observations taken at any one velocity 

 was good, and the mean values, for the different velocities tested, 

 gave points all of which were close to a smooth curve. Inter- 

 polation on this would seem, therefore, to be reliable, and the use 

 of the method to determine velocities of flow of fluids is suggested. 



