524 Mr. J. S. G. Thomas on 



of the cooling current of air in the upward flow tube Avill be 

 V fl + v c , where V a is the impressed velocity of the air-current 

 and v c the velocity of the free convection current from the 

 wire. In the downward-flowing tube, the effective velocity 

 is + (V - v c '), where vj is the velocity of the free convection 

 current from the anemometer wire therein, the 4- or — sign 

 being chosen according as Y a is greater or less than vj' 

 respectively. A little consideration w r ill show that for all 

 values of V , the galvanometer deflexion due to an impressed 

 air-velocity V in the anemometer tubes is that due to a 

 velocity equal to the difference between v e + v c ' and its initial 

 value. The deflexions respectively due to effective currents 

 Va + v c and V — vj in the anemometer tube are shown by the 

 curves for upward and downward flow in fig. 12, and the 

 resultant deflexion corresponding to any value of the air-flow,, 

 with a pair of anemometer tubes employed as explained, is 

 obviously represented by the horizontal distance between the 

 points on the respective upward and downward How curves 

 corresponding to the air-flow in question. The actual 

 calibration in the present case confirms this theory. The 

 deflexion of the galvanometer increases with increase of the 

 impressed air-flow starting from zero, attains a maximum 

 value, and thereafter decreases. The maximum air-flow in 

 the present experiment corresponded to the passage of 

 23" 7 cubic feet of air per hour, and the deflexion of the sus- 

 pended coil galvanometer shunted with 20 ohms, 2000 ohms 

 being unplugged in the constant ratio arm, was 55 divisions 

 on the scale. It is obvious, therefore, that at the velocity 

 corresponding to this relatively high flow, and the corre- 

 spondingly low temperatures of the wires, the effect of the 

 free convection current is quite appreciable on the form 

 of the calibration curve. A consideration of the curves for 

 upward and downward flow' in fig. 12 and others affords an 

 immediate explanation of the characteristics of the calibration 

 curve obtained in the present case. The horizontal distance 

 between points on the respective upward and downward flow 

 curves increases from zero with increasing air-flow, attains 

 a maximum, and thereafter diminishes in accordance with 

 the experimental result obtained. This form of double 

 exposed wire anemometer is especially suitable for use with 

 low T velocities, owing to the increased sensitiveness obtained 

 by its use. The instability apparently inherent to all hot- 

 ware anemometers employed with vertical How-tubes is 

 somewhat reduced, though not entirely eliminated, and 

 stability of deflexion is certainly secured at a lower velocity 

 than that characteristic of a single flow-tube anemometer 

 employed in a vertical position. 



