258 -Effect of a slow Current of Air flowing past Platinum. 



employed in platinum thermometry, for purposes of anemo- 

 metry. Thus wires Nos. 1 and 1L might be inserted in a 

 bridge, employing a bridge current of, say, 0*01 amp. 

 Intervening wires would be employed as a heating coil. 

 The deflexion-velocity calibration curves obtained in this 

 manner present the same main features as those discussed 

 in the present paper. With increase in the impressed 

 velocity, the deflexion increases until an upper limit is 

 reached, and thereafter decreases in the manner already 

 described. The sensitivity of the device is, however, owing 

 to the small current employed in the arms of the bridge, 

 very considerably smaller than that of the type of anemo- 

 meter described in the main part of this paper. Thus, 

 using the same electrical apparatus, with its sensitivity equal 

 to that employed throughout this work, in conjunction with 

 what may be termed the thermometric anemometer just 

 described, the heating current being 1*1 amp., the maximum 

 deflexion obtained was 10 scale divisions. The thermo- 

 metric type of hot-wire instrument has been introduced by 

 C. C. Thomas* for the measurement of gas-flow. The 

 electric energy requisite to maintain a constant difference 

 of temperature of 2°C. in a pair of differential platinum 

 thermometers situate one on each side of a heating-coil 

 arranged across the section of the tube, is measured. It is 

 clear that with certain dispositions of the thermometers and 

 heating coil, for very small values of the velocity of the gas 

 stream, the considerations advanced in the present paper 

 become of importance. Thus, provided the heating coil and 

 thermometers are suitably disposed, for low velocities of the 

 gas stream, the electrical energy necessary to maintain a 

 constant difference of temperature between the two thermo- 

 meters may decrease with increase in the impressed velocity 

 of the gas stream. 



The work detailed herein was carried out in the Physical 

 Laboratory of the South Metropolitan Gas Company. The 

 author desires to express his sincere gratitude to Dr. Charles 

 Carpenter, C.B.E., for his unfailing and inspiring interest 

 in the research, and for the ready provision of all facilities 

 necessary for the prosecution of the work. 



Physical Laboratory, 



South Metropolitan Gas Company, 

 709 Old Kent Koad, S.E. 

 20 Oct., 1920. 



* Journ. Franklin Inst. 1911, pp. 411-460; Trans. American Soc. 

 Mech. Eng\ 1909, p. 655; Proc. American Gas Inst. 1912, p. 339. 



