﻿Air Velocity by means of Hot Wire Anemometer. 573 



maximum current employed in the velocity determinations. 

 If the coils be wound bare in the form of an open grid, the 

 current-heating may easily be predetermined from the data 

 obtained from the writer's analysis of Langmuir's observa- 

 tions on the free convection of heat from small wires *. 



It may be noticed that temperature-compensation may 

 also be obtained by constructing (a, b, a, j3) of manganin, 

 and B of a wire, or combination of* wires, having the same 

 temperature coefficient as the anemometer-wire and of suffi- 

 cient current-carrying capacity to be unaffected by the 

 heating effect of the measuring- current. It is easily seen 

 that this arrangement is not as satisfactory as that already 

 discussed, since this resistance must be easily capable of 

 adjustment as different anemometer-wires are inserted in the 

 circuit. 



(iii.) Ageing and Life of Anemometer- Wires. 



The comparative immunity of the hot-wire anemometer 

 from serious corrections due to such fluctuations of room- 

 temperature as may ordinarily occur in a laboratory, is 

 secured by the use of a wire at a high temperature, this 

 being rendered possible for accurate work by the employ- 

 ment of the Kelvin Bridge connexions. The limitation in 

 the direction of high temperatures is a slow progressive 

 increase in the resistance of the wire due to "evaporation " : 

 the effect which will be referred to as " ageing " increases 

 for very thin wires, and in practice sets a lower limit to the 

 diameter of the wire which it is possible to employ for any 

 considerable length of time at about 2-g- mils. Experience 

 has shown that in the case of a wire of this diameter the 

 ao-eino- becomes distinctlv noticeable after the wire has been 

 employed to measure about 1000 velocities. In Table II. 

 are given two calibration formulae for the same wire sepa- 

 rated by 10G0 velocity determinations, showing that at the 

 high temperature of 1000° C. employed the ageing becomes 

 distinct only after extended use. For this reason it is 

 desirable to employ the wire at as low a temperature as is 

 consistent with the source of error represented by (13) being- 

 considered sufficiently small for the purpose in hand : also 

 if durability is required to use a larger wire. In precision 

 measurements of velocity it is advisable to recalibrate the 

 wire at intervals of about 500 observations; the necessary 

 whirling arm is easily improvised with materials which are 

 available in a laboratory f, and the necessary calibration only 



* Reference (1), Description of Table Y1IL. p. 424. 

 f Reference (1) p. 428, Diagram II.; also Plate 8 (a). 



