(346 Mr. J. S. Gr. Thomas on the 



A similar set of readings was made using platinum wires 

 of diameter 0*202 mm. and temperature coefficient 0*003434, 

 installed as already described in the flow-tube. In this 

 case, owing to the smallness of the resistances of the wires, 

 it was not so easy to adjust the separate resistances to 

 equality when in situ and heated by a current, as was 

 the case with the finer wires. The values of the resistances, 

 the wires conveying a current equal to 0*01 amp., were 

 0*0753! ohm and 0*0756 8 ohm for the first and second wires 

 respectively. The curves in fig. 1 show how the galvano- 

 meter deflexion depended upon the air current in the case 

 of both thicknesses of wire. For purposes of comparison, 

 the respective heating currents, in the cases of different 

 thicknesses of wire, were chosen so that the wires were 

 in the two cases, in the absence of flow, heated to approxi- 

 mately the same temperature. Table II. gives particulars 

 of the heating currents employed, and the temperatures 

 of the wires etc., in the absence of flow. Fig. 1 also gives 

 a calibration curve for the case of the thicker wire 

 (diameter 0*202 mm.), employed vertically and heated 

 by a current (3*15 amp.) so that in the absence of 

 flow, the temperatures of the vertical wires were as 

 nearly as possible equal to their respective temperatures, 

 with zero flow, when they were installed horizontally in 

 the flow-tube and heated by a current equal to 3*3 amps. 

 Fig. 1 likewise gives the form of the calibration curve 

 obtained when the anemometer wires (diameter 0*202 mm.) 

 were employed inclined at an angle of 45° to the vertical 

 and heated by a current of 3*3 amps. Considering the 

 curves obtained when using the finer wires (diameter 

 0*101 mm. and heating current 1*2 amp.), it will be 

 seen from fig. 1 that the sensitivity of the arrangement is, 

 for very small velocities of flow, considerably greater 

 when the wires are installed vertically than is the case 

 when the wires are employed horizontally. Corresponding 

 to a mean impressed velocity of about 4 cm. per sec. 

 indicated by the point P, the same deflexion is afforded 

 by the two arrangements. Over the region of velocities 

 embraced between values corresponding to those represented 

 by the points P and Q, the horizontal arrangement affords 

 the greater sensitiveness. Corresponding to values of the 

 impressed velocity greater than that represented by Q, 

 the sensitivities of the two arrangements are, within the 

 limits of experimental error, equal. With regard to the 

 greater sensitivity of the vertical arrangement for very 

 low velocities (region embraced between the origin and P), 



