the Ilot-ivire Anemometer. 515 



Paul, the galvanometer sensitiveness being appropriately re- 

 duced either by means of a shunt or by a resistance in series. 

 The results obtained for the series in which the galvano- 

 meter was shunted throughout by a resistance of 6 ohms are 

 shown in Plate X. fig. 6. Exactly analogous results were 

 obtained when 1000 ohms was employed in series with the 

 galvanometer. The results show that, in the region of low 

 air-current velocities, where the effects of the " free con- 

 vection " current are considerable, the sensitiveness of the 

 anemometer is greater the smaller the heating current 

 employed. Thus, examining the respective deflexions for 

 the same air-current amounting to 1*5 cubic feet per hour, 

 it is ^een that the deflexion when a current of 0*9 am|>. is 

 employed is considerably greater than that when a current 

 of 1-5 amp. is used in the bridge. The values of the 

 deflexions for intermediate values of the current are seen 

 to range between these two value's, "being greater the smaller 

 the heating current employed. This inversion of the order 

 of sensitiveness as related to the heating current employed, 

 compared with what occurs when higher values of the air- 

 current velocity are employed, is accounted for by the fact 

 that the balance of the bridge "with zero flow" is effected 

 under the influence of the "free convection" current. This 

 " free convection " current is greater the larger the heating 

 current employed in the bridge, and the proportional cooling- 

 effect due to any definite impressed velocity is, of course, 

 smaller the larger the "free convection " cooling effect. 

 The matter can be regarded mathematically thus : — The 

 proportional change in the effective velocity of the cooling 

 stream when a small velocity v due to the air-current is 

 impressed upon the free convection current is 



\Av + r 2 — r c v 



This is greater, the smaller the value of r c , i. e. the lower the 

 temperature to which the wire is initially heated. A com- 

 parison of the curves for 0'9 amp. and 1*1 amp. shows that 

 for higher velocities of the air-stream, the latter affords 

 the greater sensitiveness. The point at which the inversion 

 from the less to greater sensitiveness occurs, in the case of 

 any two currents, is seen to occur at a greater velocity the 

 greater the current employed. In the figure, the point of 

 inversion of the relative sensitiveness has not been reached 

 in, the case employing values of the current 1*3 and 



