588 Mr. A. M. Tyndall on the 



value of f , assumed to be the field f c in which corpuscles 

 ionize. f c was about 014 /+ . 



The author has extended the work on these smaller values 

 of y, and. has found that a variation in the radius of a 

 positively charged P, which causes a marked variation in the 

 value of the field at P, does not appreciably affect the value 

 of the critical distance (referred to in what follows as y ) at 

 which the drop in field sets in. 



Thus in Curves 1 (PI. V.) the values of f are plotted as 

 ordinates with the corresponding values of y as abscissae. Two 

 points were experimented upon, one of radius 0*062 cm. 

 (crosses) and the other of radius 0*018 cm. (circles) ; the 

 vertical scales of the curves are arbitrary, and are adjusted 

 to make the two curves identical at large values of y. 



As a result it appears that the two curves are then 

 coincident throughout ; this points to the conclusion that the 

 cause of the drop in f does not lie with P but with N. On 

 the other hand, except when the two points are a millimetre 

 or two apart, the values of these fields are independent of 

 the size of N, provided that the latter is discharging vigorously 

 before P starts. Under these conditions ?y , therefore, has 

 a value which is independent of the sizes of P and N ; it is 

 about 2 centimetres. 



Similar experiments have been carried out with a negative 

 P. These were not previously tried because P and N always 

 sparked at distances under several centimetres. Assuming, 

 however, that the field just when the spark passes is the true 

 glow field f . it has since been found that, in this case also, 

 / drops rapidly as y decreases. This is shown in Curves 2, 

 where the upper curve is the f -y curve and the lower the 

 current-?/ curve. P was of radius *026 cm. and N was a 

 very fine wire. 



Whether it is permissible or not to take the field just when 

 the spark passes as the true ionizing field, the drop in field 

 at short distances is shown to be real by the current-^ 

 curve, which is similar in form to that obtained when P is 

 positive and is giving true point discharge ; this drop in 

 current at short distances implies a falling field. 



Also by removing the plate behind P, true point discharge 

 was obtained for smaller values of y, and the same drop in/i 

 was observed. The observed currents were in this case too 

 great, because although the lines of force ending on the sides 

 of the point do not contribute to the pull, an appreciable 

 amount of current is supplied to them when the plate has 

 been removed. 



The reason why the two points sparked when P was backed 



