694 Dr. J. S. G. Thomas on 



origin — oi the calibration curve in the absence of any such 

 losses, the whole of the energy supply being utilized to heat the 

 air stream through 2° Con the gas scale, In all cases within 

 the range of velocities studied, the necessary energy supply to 

 the heating element in order that a difference of 2° C. may be 

 indicated by the thermometers is considerably greater than 

 that theoretically necessary. Owing to the absence of a 

 device for stirring the gas so as to render the temperature 

 of the gas stream uniform over a cross section, an indication 

 of 2° (J. difference of temperature in the thermometers does 

 not necessarily correspond exactly with a 2° C. rise of 

 temperature of the stream. The use of any efficient 

 stirring device was precluded by the experimental condition s- 

 in the present case and is impossible in many of the technical 

 applications of anemometry, more especially where gas 

 streams carrying dust etc. in suspension are to be measured. 



The various calibration curves show considerable departure 

 from the straight line B which would be obtained in the 

 absence of any heat losses and if the temperature indicated 

 by the second thermometer accurately represented the mean 

 temperature of the stream. The greatest departure from 

 the linear relation between the energy supply and the 

 velocity is seen in the case of curve A, corresponding to the 

 greatest distance between heating element and second 

 thermometer, as is to be anticipated owing to the heat losses 

 being greatest in this case. 



The approximate form of the curve A can be best discussed 

 by reference to the relation given by Callendar * and em- 

 ployed in the determination of the specific heat of gases and 

 vapours by the continuous-flow method. Where the velocities 

 of flow are larger than those concerned in the present series 

 of experiments, and where the heat losses are small, Callendar 



has shown that W = $Qd0 + JuW+ -^cW, where W is the 



energy supply to the heating element, S the specific heat of 

 the gas, Q the mass flow, dO the rise in temperature, hdO the 

 portion of heat loss independent of the flow, and kdO/Q, 

 the residual portion of the heat loss varying inversely 

 as the flow. Applying the appropriate value of S, and 

 employing v the velocity of flow in place of Q, the 

 relation becomes in the case of a flow of air in the present 



flow system :— W = 0-004147 vd6 + hdQ+ , repre- 



senting a series of hyperbolae, h and h being variable 

 * See e. g. Phil. Trans., A. 535. p. 390 (1915). 



