518 Mr. J. S. G. Thomas on 



the section of the pipe, is readily obtained from the above, 

 as already explained. It is seen therefore that, in general, 

 within the range of values of the electric current employed, 

 and within the range of menn velocities where the free 

 convection effect is somewhat reduced, the resistance R of 

 the wire is related to the current C and the mean velocity 

 v by a relation of the form : — 



R. = (aC-/3)-( 7 C-S)logr 



= C (a - 7 log V) — (/3 - £ log r), 



where a, /3, 7, and S are constants readily determined 

 experimentally. 



in the preceding experiments, the heated wire was 

 mounted horizontally, and the pipe itself was horizontal. 

 In order to study the magnitude of the free convection 

 current, the pipe was now placed vertically, the heated wire 

 being still horizontal. Arrangements Were made whereby 

 the current of dry air could be passed over the heated wire 

 in the pipe either in an upward or downward direction. 

 In the former case, if V rt denote the impressed velocity of 

 the air stream at any part of the wire, and v c the velocity 

 of the upward-flowing free convection current therefrom, it 

 is obvious that in the one case the effective velocity of the 

 air-current is V„ + ?v and in the other V a — r c . The method 

 of carrying out the series of experiments was exactly similar 

 to that detailed above. The pipe in which the air flowed 

 was set up vertically and had a total length of 5 feet. 

 Gauzes were arranged close to the entrance to the pipe to 

 distribute the stream across the section of the pipe. When 

 the gas flowed in a. downward direction, it was found that 

 disturbing effects were reduced to a minimum by passing the 

 air from the flow-pipe through a wide glass tube bent twice at 

 right angles and of length about 30 inches. The exit limb 

 of this tube was arranged vertically, and the gas passed 

 therefrom in an upward direction. It was found impossible 

 to secure the very steady electrical conditions with low values 

 of the velocity of flow, such as could be obtained when the 

 flow-tube was arranged horizontally. The spot of light in 

 general oscillated from 2 to 5 divisions on either side of the 

 equilibrium position, the maximum deflexion obtained in the 

 present series of observations being about 500 divisions. In 

 all' cases, however, where the velocity of flow exceeded a cer- 

 tain value, which varied with the electric current employed, 

 and was greater the greater the electric current employed, the 

 readings were extremely steady. With velocities in excess 



