172 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
(Proceedings of the Royal Society, June 10,1875). If such an instrument be used with 
an induction coil, the discharge, though often beautifully stratified, is intensely 
sensitive. The lowest limit of rapidity with which we have produced stratified 
sensitive discharges in this way is 240 breaks per second. 
We will now consider the direct experimental evidence that the current is intermit¬ 
tent in all cases in which the discharge is sensitive. This evidence is derived from the 
revolving mirror and the telephone. If the body of the tube containing the discharge 
be hidden by an opaque screen which contains a narrow longitudinal slit, and the 
image of this slit be observed in a rapidly revolving plane mirror, a series of bright and 
black bands appear whenever the discharge is sensitive, showing that there are 
striae of one current to advance upon the positions occupied by those of the reverse current, giving the 
whole column a twisted appearance. But as there is no trace, so far as my observations go, of this asso¬ 
ciation of alternate discharges when produced by the ordinary break, we seem led to the conclusion that 
a stratified discharge, on ceasing, leaves the gas so distributed as to favour, during a very short interval 
of time, a similar stratification on the occurrence of another discharge, whether in the same or in the 
opposite direction. 
“.The column of striae which usually occupy a large part of the tube from the positive towards the 
negative terminal have hitherto been described as stationary, except as disturbed by irregularities of the 
break. The coltimn is, however, frequently susceptible of a general motion or ‘ flow,’ either from or 
towards the positive pole, say a forward or backward flow. This flow may be controlled, both in velocity 
and in direction, by resistance introduced into the circuit, or by placing the tube in a magnetic field. The 
resistance may be introduced in either the primary or the secondary circuit. 
“ When the stria) are flowing they preserve their mutual distances, and do not undergo increase or 
decrease in their numbers. Usually one or two remain permanently attached to the positive electrode; 
and ,as the moving column advances or recedes, the foremost stria diminishes in brilliancy until, after 
travelling over a distance less than the intervals between the two striee, it is lost in darkness. The reverse 
takes place at the rear of the column. As the last stria leaves its position, a new one, at first faint and 
shadowy, makes its appearance behind, at a distance equal to the common interval of all the others: 
this new one increases in brilliancy until, when it has reached the position originally occupied by the last 
stria when the column was at rest, it becomes as bright as the others. The flow may vary very much in 
velocity ; it may be so slow that the appearances and disappearances of the terminal striae may be watched 
in all their phases, or it may be so rapid that the separate striae are no longer distinguishable, and the tube 
appears as if illuminated with a continuous discharge. In most cases the true character of the discharge 
and the direction of the flow may be readily distinguished by the aid of a revolving mirror. In some tubes, 
especially in those whose length is great compared with their diameter, the whole column does not pre¬ 
sent the same phase of flow; one portion may be at rest while another is flowing, or even two conter¬ 
minous portions may flow in opposite directions. This is seen also in very wide tubes, in which the sti’ise 
appear generally more mobile than in narrow ones. But in all cases these nodes or junction-points of the 
flow retain their positions under similar conditions of pressure and current; and it therefore seems that, 
under similar conditions, the column in a given tube always breaks up into similar flow-segments. 
“ These nodes will often disappear under the action of a magnetic pole. Thus if the first segment, 
measured from the positive terminal, be stationary and the second be flowing backwards (i.e., from 
— to +), a magnetic pole of suitable strength, placed at the distant end of the latter, will stop its flow, and 
the whole column will become stationary throughout. An increase in the strength of the magnet, or a 
nearer approach of it to the tube, wil 1 produce a general forward flow of the column.” 
