526 Mr. J. S. G. Thomas on 



calibration curves are shown in Plate XII. fig. 15, the ratio arm 

 employed in the bridge being 2000 ohms, and the suspended 

 coil galvanometer shunted with 12 ohms resistance. In the 

 case of this type of anemometer employed with either an 

 upward or downward flowing current of air. there is, as in 

 other cases, a region of comparative instability corresponding 

 to the portions of the calibration curves extending from zero 

 flow to a flow of about 2*5 cubic feet per hour. Thereafter 

 the readings obtained are remarkably steady. The steep 

 inclination of the succeeding part of each curve shows that 

 very high rates of flow of the gas would be necessary before 

 the instrument would cease to function efficiently as an 

 indicator of direction of flow. A deflexion of 248 divisions 

 was obtained with a rate of flow corresponding to the passage 

 of 25*4 cubic feet of air per hour, the flow-tube being 

 mounted horizontally. With a horizontal flow-tube, owing 

 to the symmetrical nature of the bridge, the zero remains 

 extremely constant for various values of the heating current, 

 and no adjustment of the balancing resistance in the bridge 

 was found necessary during a series of calibrations. For 

 the investigations of low velocities, this type of double-wire 

 anemometer possesses a further advantage over the type 

 employing one exposed wire and a second provided with a 

 protecting shield. The great sensitiveness of the double- 

 wire type at low velocities when used with horizontal flows 

 is shown by the appropriate curve in fig. 15. An examina- 

 tion of the variation of the values of the resistances of the 

 respective exposed wires with varied rates of flow showed that 

 while that of the wire first meeting the air current diminished 

 continuously from 0*5619 ohm to 0'4160 ohm, with in- 

 creasing rate of flow from zero to 23*95 cubic feet per hour, 

 under similar conditions that of the second wire increased 

 from the value 0*570 ohm with zero flow to 0*594 ohm, 

 corresponding to flow of 1*702 cubic feet per hour, there- 

 after diminishing continuously to the value 0*450 ohm, 

 corresponding to a flow of 23' 95 cubic feet per hour. The 

 explanation of this phenomenon is as follows. By its passage 

 over the first wire the air in the immediate neighbour- 

 hood of the wire is heated, and this heated air being trans- 

 ferred to the neighbourhood above the second wire reduces 

 the natural convection loss from the latter. Some heat is of 

 course convected from the latter by the stream of air, but on 

 balance, the resistance of the latter wire increases, and such 

 increase becomes greater for small values of the flow as the 

 velocity of the flow is increased, owing to the fact that with 

 such o;reater flow more heat is transferred from the first wire 



