the Hot-wire Anemometer. 519 



of these critical values, the hot-wire anemometer can be 

 employed equally well with streams flowing in an upward or 

 downward direction, as is the case with horizontal streams. 

 In the latter case, however, the steadiness of readings is 

 remarkable throughout the whole range of velocities, and 

 wherever possible hot-wire anemometers should be installed 

 in horizontal streams rather than in vertical streams of gas. 

 Morris* employed a hot-wire anemometer immersed in a 

 downward-flowing stream of air, and with regard to its 

 calibration remarks : "an unstable part of the curve will be 

 noticed at a velocity of perhaps half a mile an hour; this is 

 probably due to the unstable way in which the upward 

 gentle natural convection current is met by the downcoming 

 air-current due to the fan." The author's experience leads 

 him to believe that this instability is less when the air- 

 current is directed upward than when directed downward, 

 although even then there is still some little instability of 

 reading, but the instability disappears at a lower velocity 

 of the air-current than in the case of a downwardly directed 

 flow of air. The instability with vertical air-current is not 

 to be attributed entirely to the difference in the velocity of 

 the air-current at different points in the cross-section of the 

 pipe, as this condition also holds in the case of the horizontal 

 flow experiments where remarkably steady readings are 

 obtained, and, as pointed out, the instability is also present in 

 Morris's experiments carried out in a wind channel. It is of 

 interest to note that in the case of variable velocity across the 

 section, as in the case of flow in a pipe, as the stream flow 

 decreases towards the boundary of the pipe the cooling of the 

 wire due to the stream necessarily diminishes in the same 

 direction. The free convection current therefore increases 

 from the centre towards the boundary of the pipe. The 

 result, therefore, is that, in the case of an upward air-stream 

 flowing in the pipe, the possible difference of temperature 

 existing in the wire at the centre and boundary is diminished 

 by the existence of the free convection current, the opposite 

 effect being produced in the case of a downwardly directed 

 air-current. The diminished instability in the case of an 

 upwardly directed current of air, compared with that in the 

 case of a downwardly directed current, is probably largely due 

 to this cause. It seems probable- that the instability present 

 in the case of a vertical flow is partly due to the fact that 

 the vertical tube is subjected throughout its length to 

 the varying conditions of the surrounding atmosphere. It 



1912 ; Engineering Dec. 27th, 1912. 



