﻿Air Velocity by means of Hot Wire Anemometer, 577 



recorded data from which might be obtained the efficiency 

 of any steam-operated engine. In the case of the steam 

 turbine, the possibility of obtaining a record of performance 

 analogous to the indicator-diagram of the ordinary recipro- 

 cating engine might make the subject worthy of a special 

 investigation. 



In closing, the writer has much pleasure in thanking 

 Professor H. T. Barnes, F.R.S., Director of the Macdonald 

 Physics Laboratory, for the kind way he has facilitated the- 

 present work by every means in his power. 



SlIMMABY. 



(1) Specifications relating to the construction of the 

 linear hot-wire anemometer are given in detail ; by means 

 of the Kelvin Double Bridge connexions it is possible to 

 employ wires at high temperatures, thus making velocity 

 determinations practically independent of ordinary fluctua- 

 tions of room temperatures. 



(2) The most suitable methods of determining the con- 

 stants of the calibration formula r = i 2 + ky/Y are discussed, 

 together with evidence as to accuracy of registration for 

 velocities less than 50 cm./sec. and greater than 900 cm./sec, 

 these being the limits usually employed in calibrating' 

 anemometer- wires. 



(3) If the change of velocity just detectable by the in- 

 strument is denoted by SV, the ratio Y/8Y which defines 

 the resolving power of the anemometer is expressed in terms 

 of that of the ammeter employed in connexion with the 

 apparatus ; it is shown that with an ammeter reading to 

 0*002 ampere the resolving power of a 2J-mil wire at 

 Y = 800cm./sec. is about 140, and the change of velocity 

 just detectable is about 6 cm./sec. 



(4) Various sources of error are considered in detail ; 

 effects of variations of atmospheric pressure, humidity, and 

 temperature are dealt with and are shown to be negligible 

 under ordinary circumstances if a high-temperature wire be 

 employed. Data are given on the ageing and life of anemo- 

 meter-wires. It is shown that the anemometer measures 

 the inass-fiow of a gas ; a compensating arrangement of 

 ratio-coils is described which eliminates variations of room 

 temperature as a source of error. 



(5) Applications of the hot-wire anemometer to physical 

 and technical problems are described. 



McGill University, 

 Nov. 19th, 1914. 



Phil. Mag. S. 6. Vol. 29. No. 172. April 1915. 2 P 



