520 Mr. J. S. G. Thomas on 



was found that the effect was somewhat reduced, though 

 not entirely eliminated, by providing the tube with a double 

 lagging of thick asbestos cord throughout its length. The 

 general instability was greater the larger the heating current 

 employed, and on this account it was not possible to employ 

 the whole of the heating currents employed in the present 

 series. Experiments were, however, carried out employing 

 currents of 0"6, 0*8, 0*9, 1*0, l'l, and 1*2 amp. respectively 

 in the bridge. The sensitiveness of the galvanometer was 

 suitably adjusted in each case, so that the maximum possible 

 deflexion occurred with a downicard flow of air of about 

 4 cub. ft. per hour. Low rates of flow were alone examined, 

 and the results for downward flow of air are shown in 

 Plate XL fig. 11. Owing to the varying sensitiveness of 

 the galvanometer employed, the various series of deflexions 

 are not strictly comparable one with another. The results in 

 the case of 0'6 amp. and 0'7 amp., however, are strictly 

 comparable, as are likewise those for 1*1 and 1*2 amp. 

 respectively. In every case it was found that on increasing 

 the air-flow gradually from zero, the deflexion first in- 

 creased, attained a maximum value, and then decreased 

 until zero deflexion was again reached. Thereafter the 

 deflexion was reversed and increased continuously. Com- 

 paring the results for 0\6 and 0*7 amp., it will be seen 

 that, initially, the bridge employing the larger current is 

 more sensitive than that employing the smaller. Ultimately, 

 however, a point is reached at which the inversion of the 

 relative sensitiveness occurs, the smaller current then 

 affording the greater sensitiveness. This is clearly seen by 

 considering the respective deflexions corresponding to flows 

 amounting to 0*4 and 1'8 cubic feet per hour. The re- 

 spective points at which the maximum deflexions occur on 

 the left of the origin are indicated by P, Q, R, S, T, U, V. 

 The significance of these points is obvious. The maximum 

 deflexion occurs when the cooling effect due to the free 

 convection current is exactly negatived by the downward 

 flow of air. We may therefore conclude that the velocity 

 of the free convection current from the wire at the appro- 

 priate temperature is in any given case equal to the mean 

 velocity of the air-stream at which the maximum deflexion 

 occurs. Considering the portion of any one of the curves to 

 the left of the axis of volumes, it is seen that the ordinate 

 through any deflexion such as —100 cuts the curve in two 

 points. These two points represent a condition of affairs in 

 which the temperature of the wire is the same. The free 

 convection current from the wire is therefore the same, v c saj. 



