408 
DR. W. S. TUCKER 'AND MR. E. T. PARIS ON 
The values of U and TR are plotted in fig. 10 and a smooth curve drawn through the 
points. As the impressed air-current increases from zero the resistance of the grid 
V in cms/scc. 
Fig. 10. 
(given by R = 270 -8 + dR) rises and passes through a maximum, the curve cutting 
the U-axis at U = 2 -45 cans, per second. The maximum occurs when U = T X 2 -45 
= 1 -225 cms. per second. When U has this value, the impressed air-current balances 
the free-convection current V 0 , so that V 0 = 1 -225 cms. per second. Somewhat similar 
curves to that in fig. 10 are given in a recent paper by J. S. G. Thomas,* who used this 
method to determine the velocity of free convection from a platinum wire 0 -00784 cm. 
in diameter and carrying current of 0 -6 to 1 -2 amperes. 
The symmetrical form of the curve about the line U = Y 0 = 1 -225 suggests that 
the relation between TR and U can conveniently be represented by a formula of the 
type 
SR = SR 0 +a(U-V o y+b(U-V o y+&c., 
where £R 0 is the maximum increase in resistance occurring when U = V 0 . It was found 
that the results of the above experiment could be very fairly represented by the formula 
SR = 074-0A0 (U—l-225) 2 + 0-0044(U—U225) 4 . 
A series of points for U = 0, + 1, + 2, etc., calculated from this expression, are indicated 
* ‘ Phil Mag.’, vol. XXXIX, pp. 518-523, and PI. XI, fig. II (1920). 
