284 Dr. J. S. G. Thomas on the Forced Convection of 



the distance apart of the wires, the thermal effect on the 

 second wire due to this cause will clearly diminish as the wires 

 are brought closer together. Briefly it can be stated that with 

 the closer approach of the wires at these distances, the tem- 

 perature of the second wire becomes less dependent upon 

 the inclination of the convection current from the first wire 

 to the impressed stream. 



As the matter referred to above is of some importance in 

 the design of practical types of thermometeric hot-wire 

 anemometers * for the measurement of the flow of gases in 

 pipes, some further experiments were made with a view to 

 ascertaining how the increase of temperature indicated by 

 the second wire through which a current of O'Ol amp. only 

 was maintained depended upon the distance of this thermo- 

 metric wire from the heated wire in which a constant 

 current equal to 1*3 amp. was maintained. The resistance 

 of the thermometric wire was determined by means of a 

 Callender & Griffiths bridge, and the bridge balance could 

 be accurately adjusted to 0*1 mm. The results obtained are 

 shown in fig. 3. To prevent confusion, a few only of the 

 experimental points are given. An enlarged diagram of 

 the form of the various curves in the neighbourhood of the 

 origin is given in fig. 3 A, in which the effect of the accumu- 

 lated strata of heated a^r to which reference has been made 

 is clearly seen from the form of the curves F, G, H, and I. 

 Although no actual cooling of the thermometric wire is shown 

 by the curves B, D, E, the effect of the heated strata is not 

 entirely absent, as is shown by the forms of the curves in the 

 region oE impressed velocities corresponding to 3-4 cm. 

 per second. Certain features of the curves J, K, and L in 

 fig. 2 are clearly reproduced in the corresponding curves 

 J, K, and L in fig. 3. Attention may in particular be 

 directed to the result that, as shown by the curves K and L 9 

 the effect of an impressed stream moving with considerable 

 velocity may be to cool the thermometric wire below its 

 temperature in the absence of flow and in the presence of 

 the heated wire. The curves A' and A" correspond to 

 greater distances of separation of the wires than any shown 

 in fig. 2. Clearly, for all dispositions of the wires, an 

 appropriate velocity of the impressed stream exists such 

 that the temperature of the thermometric wire attains its 

 maximum value due to an impressed stream of air passing 

 in succession over the heated wire and the thermometric 



* See e.g. C. C. Thomas, Journ. Franklin Inst. 1911, pp. 411-460; 

 Trans. American Soc. Mech. Eng. 1909, p. 655. See also Phil. Mag. 

 vol. xli. p. 258. 



