﻿558 Prof. L. Vessot King on Precision Measurement of 



may easily be accomplished by means of a spring control- 

 In taking a measurement of velocity the key K x is pressed 

 down and the current as read by the ammeter slowly in- 

 creased until on pressing down the key K 2 a balance is 

 obtained on the galvanometer. From the reading of the 

 current i the velocity V may readily be obtained from a 

 calibration curve corresponding to the formula 



P=V+i*Y, (1} 



or from a conversion-table connecting i and V calculated 

 from the above expression. 



The ammeter employed by the writer was a Weston 

 direct-current instrument of range 2 amperes ; the scale 

 was equally graduated over this range, each division repre- 

 senting 0'02 ampere ; by estimation the current could be 

 read to 0'002 ampere. If the conditions of air-flow are 

 sufficiently steady and it is required to resolve small velocity 

 differences, the use of a Weston Laboratory Standard 

 ammeter is recommended ; the scale covers a range of 

 1*5 amperes and is uniformly graduated directly to 0"01 

 ampere ; by means of a diagonal scale it is possible to sub- 

 divide each division directly into fifths and by estimation to 

 twentieths, so that it is possible to read the current to 0*0005 

 ampere. 



The galvanometer employed was a Weston portable instru- 

 ment with jewel-bearings, of resistance 277 ohms and capable 

 of detecting a current of about 10 ~ 6 ampere ; this degree of 

 sensitivity is, in fact, ten times more than is necessary. 

 When employed in connexion with hot-wire anemometry the 

 constants of damping are very important in determining 

 the rapidity with which observations can be made. It was 

 found that equally sensitive galvanometers varied within 

 wide limits in this respect. 



A convenient form of fork suitable for holding in position 

 the anemometer-wires, and offering a minimum of disturbance 

 to the flow of air in its neighbourhood, is illustrated in fig. 2. 

 Fastened to a block of ebonite are the two arms of the fork 

 consisting of steel strips about 5 mm. in width. At the end 

 of each is soldered a small brass block drilled to receive two 

 fine needles fastened about 1 cm. apart. Threaded through 

 the eyes of these two needles is a 3-mil platinum wire 

 having its extremities firmly clamped in the brass block just 

 mentioned. The ends of the anemometer-wire are threaded 

 through these two loops and secured in position by beings 

 twisted a couple of turns around the wire ; the fundamental 

 property of the Kelvin Double Bridge already referred to 



