ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
575 
which is generally a hydrogen or nitrogen tube of very low resistance, but of consider¬ 
able diameter and length, is placed in circuit between the terminals of a Holtz machine, 
so that the discharge from the machine passes in a continuous manner through it. A 
narrow strip of tinfoil is placed round the standard-tube and connected by a wire with 
a flat piece of metal fixed to the extremity of a glass rod which is held in the hand. 
To test the intermittence of the discharge in any tube one has only to bring this piece 
of metal into contact with it.* The intermittent discharges in the tube, of whatever 
nature they may be, drive off from the flat piece of metal and through the wire in 
connexion with it electricity of a like sign to that of the pulses of free electricity 
that pass through the tube. This electricity, rushing to the tinfoil upon the standard- 
tube, produces by induction discharges within that tube, which are of course recog¬ 
nisable. Thus, as we have already said, positive intermittence in the tube under 
examination produces in the standard-tube what we have termed positive effects, i.e., 
positive discharges from the interior surface of the tube either in the form of simple 
positive luminosity, or in the more perfect form of the hollow cone accompanied by 
the truncated positive column and the intermediate dark space. Negative inter¬ 
mittence, on the other hand, produces constriction of the positive column, and in cases 
in which the action is more intense it gives the ring-terminal effect. 
Used as above described, this test leaves nothing to be desired so long as the inter¬ 
mittence in the tube under examination is positive in type. It would be difficult to 
exaggerate the sharpness with which all the details of the positive effects that have so 
often been referred to in this and our former paper come out in the standard-tube so 
soon as any positive intermittence appears in the other. But with regard to negative 
intermittence the case is different. It is, for the reasons given in the last section, 
very difficult to get good negative manifestations at the tinfoil upon the standard- 
tube. Even when they do appear they are often confused by the superposition of 
luminosity due to the discharge in the standard-tube. The best way to deal with the 
difficulty is to observe the nature of the discharge between the tinfoil and the positive 
terminal by placing the finger upon the tube as mentioned on page 571. We are then 
examining a discharge composed of a continuous and a negatively-intermittent portion 
superimposed one on the other. The latter will, of course, cause positive luminosity to 
appear within the tube beneath the finger, just as much as though the former was not 
present, and thus the presence of negative intermittence in the original tube will be 
demonstrated. With this mode of procedure the standard-tube method is probably 
as delicate a test for the existence of negative as it is of positive intermittence, 
though it still labours under great disadvantages when it is desired to learn something 
* If it is desired to augment the effect on the standard-tube a piece of tinfoil of tolerably large size 
can he placed upon the tube that is under examination, and contact can be made with it instead of with 
the naked surface of the tube. If it is desired to diminish the effect it can be done by allowing the piece 
of metal affixed to the glass rod to remain at a suitable distance from the tube under examination without 
coming into actual contact with it. 
4 E 2 
