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LXXXIII. The Thermometric Anemometer. By J. S. G.- 

 Thomas, D.Sc, A.E C.Sc, A.I.C., Senior Physicist, South 

 Metropolitan Gas Company, London*. 



Introduction. 



r |^HE therinometric type of anemometer due to 0. 0. 

 A Thomas f, which has found application in industrial 

 practice, more especially in the measurement of large volumes 

 of gases flowing in mains etc., is based upon the principle 

 that if heat is imparted from a source to a stream of gas so 

 as to raise the temperature of the stream by a constant 

 amount, then the heat energy imparted to the gas to produce 

 such rise of temperature is proportional to the rate of flow 

 of the gas. Assuming constancy of the specific heat of the 

 gas, the supply of energy to the stream to effect a given rise 

 of temperature is independent of the initial temperature 

 of the stream. Such a type of anemometer possesses the 

 desirable characteristic that it measures the mass-flow of 

 gas, and hence its indications may be made to read directly in 

 terms of standard pressure and temperature conditions. An 

 elaborate series of calibrations of this type of anemometer 

 against others of the Pitot and Venturi types has been made 

 by C. C. Thomas J, and over the range of velocities employed, 

 the indications of the three types of instruments were found to 

 be in very close agreement. The calibrations referred to 

 were carried out in a pipe of 24 in. diameter and the lowest 

 velocity corresponded with an hourly flow of 7,000 lb. of 

 air, i. e. a mean velocity of about 230 cm. per sec. The 

 straight lines obtained by plotting the rate of revolution of 

 the fan producing the flow, as abscissae against the flow deter- 

 mined from the indications of the Pitot, Venturi, and Electric 

 meters respectively, were extrapolated for low values of the 

 flow and passed through the origin of co-ordinates. Such 

 extrapolation is justifiable in the case of the Pitot and 

 Venturi meters. In the case of the electric meter, however, 

 such extrapolation is clearly unjustifiable, as under no circum- 

 stances is the whole of the heat supplied to the hearing coil 

 imparted to the gas. There are necessarily heat losses due 

 to radiation, conduction, and convection, and such losses 



* Communicated bv the Author. 



t Journ. Amer. Soc. Mech. Eng\, xxxi. pp. 1325-1340 (1909). Journ. 

 Franklin Inst , vol. 172, pp. 411-460 (1911). Proc. Amer. Gas Inst. vii. 

 pp. 340-381 (1912). 



X See e.g. Journ. Franklin Inst., he. cit. p. 433. 



