354 Dr. W. Spottiswoode on a Separator and a 



is large than when it is small. This property is doubtless well 

 known to most persons who have experimented in this field; 

 but I am not aware of any special experiments made for the 

 purpose of examining the subject. De La Rue and others 

 have noticed that with a large negative terminal the amount 

 of surface covered by the glow depends upon the strength of 

 the current; and Goldstein has mentioned that the dimensions 

 of the dark space immediately round the negative terminal 

 depend upon the size of that terminal. Pursuing the ques- 

 tion of the connexion between facility of discharge and the 

 size of the terminals, we found not only that the facility de- 

 pended less on the relative than on the absolute magnitudes, 

 but also that the size of the positive may be reduced indefi- 

 nitely without interfering with the discharge. Indeed, when 

 we reflect on the appearance of the discharge under ordinary 

 circumstances, and consider that, whatever be the size or 

 shape of the positive terminal, the discharge at that end 

 always proceeds as from a point, then we are almost brought 

 a priori to the conclusion that the positive terminal itself may 

 be reduced to a point without affecting the discharge. The 

 reverse, however, is the case with the negative terminal ; for, 

 without entering more deeply into the question, it is clear 

 that, if we reduce the surface of that terminal below the extent 

 which would have been covered by the glow when unrestricted, 

 we thereby interfere with the natural development of the dis- 

 charge. In accordance with this consideration, we found that, 

 when one terminal was small, in that one the discharge which 

 was positive passed freely, while the opposite discharge 

 failed altogether to pass ; and thus in this case the problem, 

 as it were, solved itself. The same results were obtained even 

 when one terminal was reduced to a mere point. Acting on 

 this principle, we inserted a tube, having one very small and 

 one large terminal, in series with the tube which was the sub- 

 ject of experiment ; and by means of it as an auxiliary tube 

 we succeeded in shutting off one set of currents from tho 

 main tube. 



Another, although not more successful, accessory for the 

 same purpose was found in a tube furnished with a series of 

 diaphragms each having a small aperture, with a tubular pro- 

 cess leading from it and coincident with the axis of the tube. 

 This form of tube is known as a Holtz tube; and it possesses 

 the property of allowing the passage of discharges directed 

 towards the tubular processes, but of impeding the reverse 

 discharges. Speaking more accurately, each of these aper- 

 tures offers resistance to the passage of the electricity in 

 either direction, but more so to that in which the end of the 



