256 Dr. J. S. G. Thomas on Thermal Effect of a slow 



the series were also heated, and in the case of the broken- 

 line carves, wires subsequent to the pair inserted in the 

 bridge were not heated. The heating current, both in the 

 anemometer wires and in the heating coil, were throughout 

 adjusted to 1*1 amp., and the galvanometer shunted with 

 10 ohms. The curves in fig. 7 are therefore, strictly comparable 

 with those given in fig. 6. Considering first the full-line 

 curves, which were obtained with all wires of the series heated, 

 it will be seen that, while the ultimate sensitivity of the 

 anemometer device for comparatively large velocities is 

 greater, the later the wire employed in conjunction with the 

 first is situated in the sequence of wires, yet this is not so 

 for small values of the impressed velocity. Thus, whereas 

 the respective deflexions corresponding to an impressed 

 velocity of 8 cms. per sec. are 48, 150, and 200 when the 

 2nd, 5th, and 11th wires are used in the bridge in conjunc- 

 tion with the 1st wire, the deflexions corresponding to an 

 impressed velocity of -4 cms. per sec. are respectively 32, 82, 

 and 57. It is clearly seen that for low values of the im- 

 pressed velocity the arrangement employing the 5th wire 

 in conjunction with the 1st is more sensitive than that 

 employing the 11th in conjunction with the 1st. The 

 explanation of this phenomenon is clear from a consideration 

 of the curves in fig. 3, wherein it will be seen that initially 

 the temperature of No. 5 wire rises more rapidly than that 

 of No. 11 wire, although, as seen from fig. 2, wire No. 11 

 attains the highest ultimate rise of temperature of the whole 

 sequence of wire. It is clear from fig. 3 that, under the 

 conditions specified, wire No. 5 used in conjunction with 

 wire No. 1 affords the hot-wire anemometer employing one 

 wire in each appropriate arm of the bridge possessing the 

 maximum sensitivity in the region of very low velocities. 

 From fig. 7 it will be likewise seen that, in accordance with 

 anticipations advanced on page 249, the sensitivity of the 

 anemometer device employing wires Nos. 1 and 2 is greater 

 when subsequent wires are not heated than when such wires 

 are heated. It will also be noticed that while this is also 

 true of the anemometer device employing wire No. 5, in 

 conjunction with wire No. 1, for values of the impressed 

 velocity greater than about 6 cms. per sec, a reversal of the 

 relative sensitivities occurs helow this velocity, the greater 

 sensitivity being then shown by the device in which subse- 

 quent wires are heated. The case of wires Nos. 1 and 5 

 being employed differs essentially from the case in which 

 wires Nos. 1 and 2 are employed. As already pointed out, 

 the extreme wires in a sequence are subject, as shown in figs. 

 4 and 5, to what may be termed an " end effect/' The use 



