On the Thermometric Anemometer. 689 



become relatively more important the less the velocity of tbe 

 stream. It is clear that if the thermometers be symmetri- 

 cally disposed with regard to the heating coil employed, 

 no value of the heat suppy to the latter would be such as to 

 maintain the desired difference of temperature between the 

 two thermometers in the absence of flow. If, however, the 

 second thermometer be disposed closer to the heater than the 

 first thermometer, in the absence of flow, the value of such 

 heat supply is perfectly definite and determined principally 

 by radiation and convection losses from the channel in which 

 the heater is placed. The effect of the predominance of such 

 heat losses for low velocities appears to have been overlooked 

 in the papers alreidy referred to *. It will be found that 

 the curves obtained by plotting the rate of flow of gas as 

 abscissae against the rise of temperature due to a constant 

 supply of energy to the heating coil as ordinates, is not 

 asymptotic to the ordinate axis. Owing to the heat losses 

 referred to, there exi>ts a finite maximum temperature rise 

 through which the stream is heated by a constant supply of 

 energy. The value of the velocity of flow at which such 

 maximum rise of temperature occurs is conditioned by a 

 variety of factors, the size of pipe, heat insulation of pipe, 

 etc. For velocities of flow less than this critical value, 

 decreasing velocity is accompanied by a decreasing rise 

 of temperature. Attention has been directed to the pheno- 

 menon in a previous paper f. Its consideration is of some 

 consequence in technical practice, e.g., in the design of heat 

 interchangers or regenerator furnace-settings for the attain- 

 ment of a maximum temperature in the gases to be heated. 



In the technical form of thermometric meter, the ratio 

 of maximum flow to minimum flow for corre-t registration 

 is about 15:1, and by employing a second pair of tempera- 

 ture-difference resistances this ratio can be increased to 

 about 60 : 1. Velocities quite commonly occurring in tech- 

 nical practice are of the order of a few cms. per sec, and 

 upwards, and it is important to determine the forms of the 

 calibration curves of a low capacity anemometer of the 

 thermometric type for such velocities, and more especially 

 the variation in such calibration cuiwes accompanying a 

 variation of the heat losses due to conduction and radiation. 

 The present paper details some of the results obtained in the 

 course of such an investigation. 



* See e. q. Journ. Franklin Inst., loc. cit. p. 447. 

 t Phil. Ma-, vol. xli. p. 258 (1921). 



Phil. Mag. S. 6. Vol. 43. No. 256. April 1922. 2 Y 



