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



deflexions occurred when various heating currents were 

 employed are shown by the broken curves in figs. 4 and 8 

 in the cases of directional hot-wire anemometers D4 and 

 D5 respectively, employing wires of respective diameters 

 0*101 mm. and 0*202 mm. In addition to determining 

 the velocity corresponding to the maximum deflexion, it 

 appeared desirable to ascertain the values of the velocities 

 at which the resistance of the second wire attained its 

 maximum value in the case of each heating current employed 

 in the bridge. In the case of wires of: diameter 0*101 mm., 

 this was readily done from the results plotted in fig. 5. The 

 experimental points in the neighbourhood of the respective 

 maxima were plotted on an enlarged scale, and the maximum 

 point accurately determined by joining the mid points of 

 chords drawn parallel to the axis of velocities. The results 

 obtained in the case of wires of diameter 0*101 mm. are 

 set out in Table IV. herewith. 



Referring to Table IV. and fig. 4, it will be noticed that 

 the possible range of velocities, — determined by the impressed 

 velocity of the stream corresponding to the maximum 

 deflexion — within which the directional anemometer may be 

 employed in a quantitative manner can be very considerably 

 extended by increasing the initial temperature to which 

 the wires are heated. Thus, while with wires of diameter 

 0*101 mm., employing a heating current equal to 0*4 amp., 

 the maximum deflexion occurs at an impressed velocity of the 

 air-stream equal to 2*57 cm. per sec, the maximum deflexion 

 occurs at an impressed velocity equal to 27*7 cm. per sec, when 

 the heating current is increased to 1*5 amp. The initial tem- 

 peratures of the wires in these two cases were respectively 

 36° 0. and 826° C. in excess of atmospheric. From fig. 8 

 it will be noted that the range of application can be still 

 further extended by employing thicker anemometer wires. 

 Thus in the case of wires of diameter 0202 mm. the 

 velocity at which the maximum deflexion occurred was 

 increased from 4*95 cm. per sec. to 35*2 cm. per sec, 

 when the heating current was altered from 2*0 amps, to 

 3*8 amps. The corresponding initial temperatures of the 

 wires were 108° C. and 735° C. respectively above atmo- 

 spheric The sensitiveness of the anemometer employing 

 the finer wires is somewhat greater than that of the arrange- 

 ment employing the thicker wires, the wires being in each 

 case raised to the same initial temperature above atmospheric 

 temperature for purposes of comparison. In the case of the 

 finer wires (diameter 0*101 mm.) the initial excess tempe- 

 rature is given with considerable accuracy by the relation 



