﻿364 Prof. L. Vessot King on Precision Measurement of 



For the study of velocities lower than .17 cm./sec. the 

 writer has found the whirling-table method unsatisfactory, 

 as elaborate precautions have to be taken against the dis- 

 turbing effect of draughts, perfect air-stillness being very 

 difficult to secure in an ordinary laboratory, while the 

 correction for " swirl " is not easily made and theoretically 

 and practically is proportionally greater at small velocities 

 than at large ones*. A more promising method of calibra- 

 tion is to employ a horizontal or vertical rail along which a 

 carriage can be driven at a measured velocity and arranged 

 automatically to move smoothly backwards and forwards 

 with constant speed over its range ; the rail need only be 

 moderately long (2 or 3 metres), as the anemometer has no 

 appreciable lag and little more than a second is required to 

 obtain a balance on the galvanometer f. 



At high velocities the linear relation (1) was tested for 

 velocities as high as V = 900 cm./sec, the usual upper limit 

 at which anemometer-wires are calibrated by the writer in 

 practice. From experiments on the flow of air between 

 parallel planes, it appears that the calibration formula may 

 be extended to velocities as hio-h as 2800 cm./sec, making 

 use of a 3-mil wire. An example of a test on this point is 

 discussed under fig. 4 : the distribution of velocity of a 

 stream of air issuing from a channel 0'75 mm. in width was 

 measured at intervals of 0*05 mm. When the resulting 

 curve, which attained a maximum of about 2800 cm./sec, 

 was integrated to obtain the total flow in cm. 3 per sec, 

 a fair agreement was obtained with the value of the total 

 flow measured from the rate of fall of the gasometer, the 

 pressure-difference over the length of the channel remaining 

 the same in the two cases. Above this velocity it was found 

 difficult to work with wires of the usual length of about 

 5 cm,, as transverse vibrations are liable to be set up which 

 invalidate the readings and tend to break off the potential 

 leads, while the tension on the wire required to destroy 

 synchronization with the free period of a stretched wire in 

 an air-current is near the breaking-point of the wire. The 

 upper limit of velocity measurement might be increased 

 almost indefinitely by shortening the wire or by using 

 stouter wire; this step results, however, in diminished 



* Reference (1), Description ot Diagram II., p. 428. 



f For horizontal calibration a photometer-bench and carriage could 

 well be used, while for vertical work the anemometer may be mounted, 

 suitably counterpoised as one of the weights of an ordinary Atwood's 

 Machine, the moving .system being electrically driven. An excellent 

 design for the purpose is the moving lamp photometer described by 

 Trowbridge and Truesdell (Phys. Rev. iv. p. 290, Oct. 1914). 



