280 Prof. A. P. Chattock ami Mr. A. M. Tyndall on the 



the two points were kept in line with one another, the 

 radius of the point being the same as before (0*031 cm.). 



In a third set points with radii lying between 0'062 and 

 O'OOI cm. were used. 



The results of the second set for positive and negative are 

 plotted in Curves I. and II. respectively, as they were obtained 

 for the widest range of conditions ; but all three sets agree 

 closely in their main features. 



Positive Discharge from P. 



Carves I. (PI. IV.) apparently exhibit all the three cases, 

 a, b, c. described on p. 278. Each curve is made up of a 

 steep part S, and a nearly horizontal part H, joined by a 

 curve. Somewhere in this curve or below it P began to glow, 

 so that along S the discharge was dark and along ti luminous. 

 The exact position of the beginning of the glow was not 

 easy to determine, as the light nearly always grew gradually 

 from small beginnings, and though the observations were 

 made in the dark it was extremely difficult to tell when it 

 first became visible. In this respect the glow differed 

 markedly from that at a negative point which started 

 suddenly. 



S and H correspond closely with cases a and b respectively. 

 For besides the fact that the discharge in S is dark and 

 in II luminous, the values of f for H rise as they should 

 do when the number of external ions is reduced by increasing 

 y ; and f Q reaches its maximum value when N is removed 

 altogether (y = oo), H then corresponding with case c. 



When y = 0'2 cm. f has been reduced about 2*3 times, 

 the discharge being still apparently ordinary point discharge; 

 but if the points are put 0*15 cm. apart streams of small 

 sparks result *. 



Provided the frequency of these sparks is not too great 

 we may write 



/o=KJ, 



where t is the time counted from the last spark and K is a 

 constant. If P is the average pull on the point, we have 



\/P = apparent value of f 



_ maximum value of / 



* It is possible by getting the discharge to start with y greater than 

 0*15, and then decreasing it to get point discharge at this distance also. 

 This was done in the case of the readings discussed below. 



