﻿Air Velocity hy means of Hot Wire Anemometer. 575 



Section 5. Notes on Various Applications of the 

 Hot- Win e Anemometer . 



The hi oil resolving power, comparative freedom from 

 serious corrections, together with extreme sensitiveness at 

 low velocities, make the linear anemometer a very suitable 

 laboratory instrument for use in studying various problems 

 of gas-flow. In particular the instrument has recently been 

 employed by the writer in a detailed investigation on the 

 flow of air between parallel planes, with especial reference 

 to the study of criteria of stability of laminar flow ; in (act, 

 the svstem of precision anemometiy described in the present 

 paper was evolved with special reference to this problem. 

 The detailed analysis of velocity gradients furnishes a new 

 method of attacking problems of gaseous viscosity, while 

 investigations on the heat-loss from a wire at different 

 velocities, pressures and temperatures promise to throw much 

 light on phenomena relating thermal conduction in cases. 



In the course of the investigation referred to, and which 

 it is hoped to publish shortly, it was found necessary to take 

 many thousand observations ; although it was found possible 

 to make as many as 100 velocity determinations an hour,, 

 and to reduce them very rapidly in the manner already 

 indicated, it is easily seen that more extended investigations 

 of this type will require very considerable routine labouiv 

 The equipment necessary for work of this kind is not usually 

 available in a physical laboratory, and the prosecution of 

 research of this type is better suited to the resources and 

 personnel of modern aerotechnical laboratories. In this 

 field the linear anemometer described would seem to have a 

 wide held of usefulness as a standard instrument, the need 

 for which is several times emphasized in the 1912-13 Tech- 

 nical Report of the Advisory Committee for Aeronautics *► 

 There is no doubt that a compilation of results obtained in 

 this way would assist very materially in the development of 

 a rational theory relating to many problems of aerodynamic 

 resistance. A recording form of instrument is now under 

 investigation by the writer, and it is hoped by this means to 

 very materially lessen the labour of taking observations ; it 

 is also hoped to do away with the necessity of calibrating 

 anemometer-wires by arranging that previously calibrated 

 wires be issued from a reliable firm of instrument-makers l 

 the Kelvin Bridge connexions makes the insertion of such 

 calibrated wires, with potential terminals already fused in 

 place, possible without risk of introducing error due to im- 

 perfect contacts. 



* Darling: & Son, London, 1914, President's Report, p. L6. 



