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



air-stream corresponding to the maximum galvanometer- 

 deflexion when various heating currents were employed 

 in the bridge could not be accurately determined from 

 the curves shown in fig. 4, on account of the extremely 

 small variation of such maximum deflexion accompanying 

 alteration in the value of the impressed velocity of the 

 air-stream in this region. The value of such impressed 

 velocity, corresponding to the maximum deflexion, could, 

 however, be very accurately determined by increasing the 

 sensitivity of the galvanometer employed, whereby the rate 

 of variation of the deflexion in the region of its maximum 

 value could be increased as desired. For this purpose, 

 therefore, the suspended coil galvanometer employed was 

 substituted by a similar galvanometer of equal resistance 

 whose sensitivity could be suitably adjusted by means of 

 a shunt. Employing a shunt of 4 ohms, and a definite 

 heating current in the bridge, the approximate value of 

 the impressed flow of air corresponding to the maximum 

 deflexion was determined in the ordinary manner. Torsion 

 was now applied to the suspension of the galvanometer 

 until the spot returned to the zero of the scale. (The 

 desirability of substituting the galvanometer employed for 

 measuring comparative deflexions by another, arises from 

 this necessity for subjecting the galvanometer suspension 

 to torsion.) The sensitivity of the galvanometer being now 

 suitably increased by employing a suitable galvanometer 

 shunt, and a constant predetermined current being main- 

 tained in the bridge, the flow corresponding to the 

 maximum deflexion could be accurately determined. The 

 bridge arrangement employed is such that no appreciable 

 alteration in the heating current in the anemometer wires 

 accompanies the alteration in the equivalent resistance of 

 the galvanometer. This is readily seen from a general 

 consideration of the relative magnitudes of the various 

 resistances in the bridge. Fig. 7 shows the bridge arrange- 

 ment employed, the values of the respective resistances 

 and currents being as indicated, and the first and second 

 anemometer wires denoted by the ordinal numbers within 

 brackets. 



It is well known that 



' y " G(a + b + * + ft) + (6 + a){a + 0y 

 and as a and b are throughout, in the present experiments, 



