652 Mr. J. S. G. Thomas on the 



explainable in the following manner. The passage of a 

 slow stream of air over the wires is accompanied, as already 

 explained *, by the convection of heat from the neigh- 

 bourhood of the first wire to that of the second, whereby 

 the second wire attains a higher temperature than in the 

 absence of the impressed flow of air. The stream of air 

 is, however, owing to imperfect shielding of the second 

 wire by the first, operative in producing some cooling 

 effect upon the second wire, and the resultant temperature 

 of the second wire is determined by the net effect of 

 these opposing tendencies. An impressed velocity can be 

 imagined of such magnitude that with suitable disposition 

 of the wires the temperature of the second wire remains 

 practically unaffected by the stream. Such a condition 

 of affairs is represented by the point E, in fig. 1, and a 

 number of other illustrations are to be seen in sub- 

 sequent curves (see fig. 5). The initial difference in the 

 temperatures of the two wires when mounted horizon- 

 tally — an arrangement in which the sensitivity depends 

 mainly upon an increase of the resistance of the second 

 wire — is therefore small, compared with the corresponding 

 difference in temperature of the wires for the same impressed 

 velocity, when mounted vertically, as in this latter case 

 any reduction of temperature of the first wire, due to 

 cooling by the stream of air, is not diminished by a heating- 

 effect operative in the counter direction. In the absence 

 of such considerations, considering merely heat convected 

 by the stream from the first wire to the second, one would 

 anticipate a greater difference of temperature between the 

 wires for a given low value of the impressed velocity of 

 the air-stream, when such difference of temperature is 

 attributable principally to cooling of the first wire, than 

 when it is mainly due to heating of the second wire ; for in 

 the latter case, owing to loss of heat by the stream of fluid 

 during its passage from the first wire to the second, and 

 owing to the fact that the whole of such residual convected 

 heat is not transferable, under the conditions of the expe- 

 riment, to the second wire, the rise of temperature of the 

 second wire due to this cause is necessarily less than the fall in 

 temperature of the first wire, the wires being assumed similar 

 in all respects. Now it will be remarked from fig. 2, that 

 when the wires are mounted horizontally, corresponding to 

 impressed velocities of the air-stream of from 1*6 cm. per sec. 



* Proc. Phys. Soc. vol. xxxii. Part 3, pp. 196-207 (1920). 



