Directional Hot-Wire Anemometer. 643 



balance of the bridge. The current of air \\;is derived 

 from a weighted gas-holder of 5 cubic feet capacity, pro- 

 vided with automatic pressure compensation, ami was 

 adjusted to any desired value by means of a uterometer 

 cock. The air was dried by passage through a column 

 of calcium chloride. The magnitude of the air-flow was 

 ascertained by means of a wet gas-meter, by Sugg, of 

 1/12 cub. ft. capacity, this being standardized by means 

 of the 1 12 cub. ft. bottle prescribed by the Metropolitan 

 Gas Referees. The pressure at the meter outlet did not 

 exceed 0*15 in. water. Further details of the experimental 

 method may be derived from the papers already referred to. 



(1) Comparative sensitivity of the directional hot-wire anemo- 

 meter when the wires are mounted (a) horizontal/// and 

 (b) vertically in a horizontal flow-tube. 



The horizontal or vertical condition of the Avires could, if 

 desired, be very accurately ascertained by the electrical 

 method devised by the author*. In general the horizontal 

 position was sufficiently accurately determined by the hori- 

 zontally of a generating line forming a continuation of 

 the line joining the centres of the holes -through which the 

 wires were inserted into the flow-tube, and ruled on the 

 tube. The vertical position was obtained therefrom by 

 rotation of the flow-tube through 90° as indicated by an 

 index moving over a scale engraved on the circumference of 

 the flow-tube. 



Results and Discussion. — The following particulars refer 

 to directional anemometer D4a used in these experiment- : — 



Diameter of tube 2*0534 cm. 



Diameter of wire employed (mean \ 



of 7 readings, differing by not \ 0*101 mm. 



more than 0002 mm.) .... J 

 Distance between wires .... 0*1 cm. 

 Temperature coefficient of wires . 0*003253 

 R , 1st wire (current 0*01 amp.) . 0*2556 ohm. 

 R , 2nd wire (current 0*01 amp.) . 0*2517 ohm. 

 Heating current in bridge . . . 1*2 amp. 



The respective volumes (at 0° C. and 7b'0 mm.) are 

 converted to the equivalent mean velocities of the air- 

 stream (calculated at 0° 0. and 7(50 mm.) by multiplying 

 the former, as given in cubic feet per hour, by 2*374. The 

 galvanometer sensitiveness was suitably reduced by shunting 

 with 6 ohms. The results are set out in Table I. 



* Proc. Pbys. Soc. 1920, vol. xxxii.Tait 5, pp. 291-^14. 



