﻿Precision Measurement of Air Velocity. 557 



connexions shown in fig. 1; the ratio-coils are adjusted so 

 that a/b = a/l3, in which case a fundamental property of this 

 arrangement is that when a balance is obtained on the 



Fig. 1. 



Diagram of Kelvin Bridge Connexions employed in Precision 

 Hot-Wire Anemometrv. 



galvanometer, A/B=a/& = a//3 independently of all connect- 

 ing- or contact-resistances. The resistances A and B refer 

 respectively to the resistance of the anemometer-wire between 

 potential terminals permanently fused to the wire and to 

 that of a manoanin resistance. The resistances a and b were 

 made equal and about 500 ohms, while a and j3 were 

 adjusted to equality at about 250 ohms. In order to protect 

 the anemometer-wire from accidentally burning out, a key 

 K x was inserted by means of which it was always short- 

 circuited except when observations were actually being taken ; 

 a double-contact key K 2 was inserted in the galvanometer 

 circuit in such a way that contact was first made through a 

 high resistance in the preliminary adjustments ; it was also 

 found convenient to connect the galvanometer through an ad- 

 justable shunt. The resistance B was constructed of No. 23 B. 

 and S. gauge manganin wire wound non-inductively on an 

 asbestos frame so as to dissipate a maximum amount of heat ; 

 its resistance as measured between potential terminals soldered 

 to the wire was adjusted to about four times that of the 

 anemometer-wire at room temperature. By means of a fine- 

 adjustment rheostat R, the current in the anemometer-wire 

 can be adjusted until a balance is obtained on the galvano- 

 meter. It is advisable that the rheostat be always readjusted 

 to the position of minimum current to avoid overheating the 

 wire should the velocitv of air-flow suddenly diminish ; this 



