Heat from a Pair of fine heated Wires. 281 



equal to ij'28 cm., 5*31 cm., 4*32 cm., 3*31 cm., and 2'31 cm., 

 indicated by the curves A, B, C, D, and E respectively, it is 

 seen that this group is characterized by a gradual increase in 

 the maximum temperature attained by the second wire of the 

 pair as the distance apart decreases. The initial portions of 

 the curves A, B, and C approximate to the form of the 

 initial portion o£ the curve L, obtained when the second 

 wire alone was heated. Moreover, the initial cooling effect 

 experienced by the second wire when the velocity of the 

 impressed stream is small is reduced as the distance apart is 

 diminished until it is replaced by an initial heating effect in 

 the case of the curves D and E. This is in accordance with 

 anticipations based on the consideration of the resultant 

 thermal effect experienced by the second wire as being con- 

 ditioned by two opposing tendencies, viz. the direct cooling- 

 effect due to the impressed stream, and heating effect arising 

 from the forced convection of heat by the stream from the 

 first to the second wire*. It will be noticed that corre- 

 sponding to the higher values of the impressed velocity of the 

 stream, portions of the curves A, B, C, D, and E are, within 

 the limit of experimental error, common to the several 

 members of this group. Moreover, as the distances apart of 

 the wires become less, the value of the impressed velocity at 

 which the respective curves unite is reduced. This result 

 is readily deduced from a graphical consideration of the 

 direction of the resultant stream due to free and forced 

 convection from the wires as previously explained t- The 

 second group of curves is constituted of curves E, F, G, and 

 H, the curve E being included as affording a transition from 

 the first to the second group. In this group, the maximum 

 temperature attained by the second wire diminishes as the 

 distance apart of the wires diminishes. The respective 

 values of the maximum increase of resistance characteristic 

 of the various curves are plotted and lettered appropriately 

 to the respective curves in fig. 4, which is discussed later. 

 It will be noticed that, as shown by the curve H, when the 

 distance apart of the wires is 0814 cm., for no value of 

 the velocity of the impressed stream does the second wire 

 become heated above its initial temperature. The charac- 

 teristic feature of this group of curves arises owing to the 

 respective distances apart of the wires in the cases in question 

 being such that, in the absence of an impressed flow, the 

 temperature of each wire is determined in part not only by 

 the heated air accumulated in the upper portion of the cross 



* Proc. Pkvs. Soc. vol. xxxii. Part iii. p. 203 (1920). 

 t Phil. Mag. vol. xli. p. 217 (1921). 



