251 Dr. J. S. Gr. Thomas on Thermal Effect of a slow 



velocities o£ the air stream it will be seen from fig. 6 that 

 in the cases where 4 or 5 wires were employed in the re- 

 spective arms of the bridge, the deflexion is initially negative 

 and increases numerically as the magnitude of the impressed 

 velocity of the air stream is increased, until a maximum 

 value of the deflexion is attained, the deflexion thereafter 

 decreasing, attaining a zero value and thereafter increasing 

 in the manner already described. A ready explanation of 

 this apparently anomalous result is readily afforded by a 

 consideration of the curves in fig. 3. It will be seen that, 

 initially, the temperatures of wires Nos. 2 to 5 increase 

 more rapidly than those of succeeding members of the 

 series. The temperatures of Nos. 6, 7, 8, and 9, it will be 

 noted, increase initially very slowly. It will be seen, there- 

 fore, that whereas for low values of the impressed Telocity 

 the rise in temperature of the wire No. 2 is always greater 

 than that of wire No. 1, and the rise in temperature in the 

 group of wires Nos. 3 and 4 greater than that of the group 1 

 and 2, by the introduction of wire No. 6 the mean rise of 

 temperature of the group containing it may be initially less 

 than that occurring in the^ corresponding group inserted in 

 the bridge. A similar argument holds in the case of groups 

 of wires in which subsequent wires are included. This 

 reversal of the initial direction of deflexion is shown in fig. 6 

 in the respective cases where four and five wires are employed 

 in each arm of the bridge, as already explained. That the 

 effect was not attributable to any abnormality of the setting 

 of any of the wires in the flow-tube, was readily proved by 

 reversing the order of the wires in the flow-tube. The 

 calibration curve so obtained, employing five wires in the 

 respective arms of the bridge, is shown by the broken curve 

 5R in fig. 6. This, while not identical with the correspond- 

 ing full-line curve — a result due to want of exact equality of 

 the respective wires — reproduces the characteristic feature of 

 reversal of the initial deflexion. Zero deflexion corresponds, 

 of course, to the attainment of the same mean rise of tem- 

 perature, due to the impressed air stream, in the two arms of 

 the bridge. Fig. 6 indicates the attainment of such a state 

 of affairs, in the case of five wires being employed in each 

 arm of the bridge, for a value of the impressed velocity of 

 the air stream equal to about 2*7 cms. per sec. The curves 

 given in fig. 3, which are based on results obtained about 

 two months prior to tho^e shown in fig. 6, the wires having 

 been intermittently employed during this period, indicate 

 that zero difference in the mean rise of temperature corre- 

 sponds to a value of the impressed velocity of the air stream 

 equal to about 2'2 cms. per sec. Exact agreement is not to 



