HEAT FROM SMALL CYLINDERS IN A STREAM OF FLUID. 
407 
For the same wire the current i required to bring the wire to the same temperature 
is given by a relation of the form 
f = V + . (80) 
where v and & are the constants of the instrument determined either by calculation 
or by direct calibration ; in practice there is some advantage in making use of a 
dynamometer form of ammeter in which the scale reading is proportional to i 2 , provided 
it is sufficiently accurate ; the range of readings can be much increased by arranging 
the instrument so that the scale has a suppressed zero. 
Another point connected with hot-wire anemometry may be mentioned here. The 
constants of the instrument or its calibration-curve determined by means of a rotating 
arm represent determinations of velocity appropriate to stream-line motion ; when 
employed to measure turbulent flow the velocity measured by the hot-wire instrument 
may be referred to as the effective velocity. The question then arises as to whether this 
measure is a consistent one and is independent of the diameter of the wire. This 
point was settled in the course of experiments carried out by Prof. A. M. Gray( oIJ ) and 
the writer on the distribution of air-velocities in the neighbourhood of rapidly rotating 
cylinders. The velocity gradient in the neighbourhood of a rapidly revolving wooden 
pulley was measured by the use of an anemometer fitted with a 3-mil wire and also 
under the same conditions with a 5-mil wire. It was found that the velocity 
gradients agreed within the limits of experimental error. It would thus seem possible 
to obtain a consistent measure of turbulent flow under conditions where other methods 
of measurement would be inadequate. 
Section 18. Note on the Applications of the Hot-Wire Anemometer. 
The advantages of the anemometer designs and connections described under 
Diagram III. in the establishment of a standard system of anemometry may be briefly 
stated as follows :— 
(i.) The use of the Kelvin bridge connections makes it possible to standardize and 
calibrate anemometer wires at a central laboratory independently of the remainder of 
the apparatus with which the wire is to be employed. 
(ii.) These connections also enable the wire to be heated to a high temperature and 
so makes the determinations of velocity practically independent of ordinary variations 
of room temperatures. 
(iii.) The use of the linear anemometer makes it possible to establish a consistent 
measure of turbulent flow. 
(iv.) The linear anemometer makes it possible to analyse sharp gradients of velocities 
without disturbing the flow at the point of measurement. 
( 50 ) Of the Department of Electrical Engineering, McGill University. 
