the Hot-wire Anemometer. 527 



to the neighbourhood of the second, and, moreover, this heated 

 current of air suffers less fall of temperature in transit from 

 one wire to the other, on account of the diminished time of 

 transit. With still larger rates of flow, the cooling effect 

 due to the stream more than counterbalances the increase of 

 temperature due to convection of heat from one wire to the 

 other, and thereafter the resistance of the second wire falls. 

 It is clear, therefore, that for low velocities of flow, owing 

 to the heating of the second wire as explained above, 

 a double exposed wire anemometer of this type may be a 

 far more sensitive instrument than the type in which the 

 second wire is provided with a protecting shield so that it 

 experiences no heating or cooling from the stream of air. 

 This point is discussed in detail in the Proceedings Phys. 

 Soc. 1920. 



Use of the Hot-wire Anemometer with Gases other than Air ; 

 the Glass-coated Hot-wire Anemometer. 



The measurement of the velocity of a stream of air in a 

 pipe, although frequently occurring in technical practice, is 

 by no means that most frequently called for, at least not in the 

 author's experience. The hot-wire anemometer in the original 

 form described on pp. 505-507 can be readily applied to 

 the case of the anemometry of gases, such as C0 2 , N 2 , 2 , etc., 

 which are not adsorbed by the heated platinum wire nor 

 catalytically decomposed thereby. The use of a bare heated 

 platinum wire, particularly if the temperature is somewhat 

 elevated, is practically impossible with a gas such as hy- 

 drogen, carbon monoxide, and gaseous mixtures such as 

 coal-gas, etc. The readings obtained in such cases are 

 extremely unsteady and unreliable. The sphere of useful- 

 ness of the hot-wire anemometer is considerably extended 

 by employing as the sensitive wire a fine platinum wire on 

 to which is fused a fine coating of glass *. This is easily 

 effected by drawing down a piece of glass tube so that it is 

 very fine in the wall, and so that its internal diameter is very 

 slightly in excess of the wire. The glass is slipped over the 

 wire, and can then, with practice, be readily fused on to the 

 wire so as to form a surprisingly uniformly thick coating by 

 heating the two cautiously in a small blowpipe flame to just 

 past the softening point of the glass, care being exercised 

 that the glass is not heated to too high a temperature and 

 not exposed to the elevated temperature for such a length of 



* Thomas, Journ. Soc. Chem. Ind., he. cit. 



