170 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
the machine with one terminal of the exhausted tube, and the other terminal of 
the machine with the outside of the tube, as in Plate 15, tigs. 2 and 3. If the current 
be then permitted to pass between the terminals of the machine by leaping a 
considerable distance (say half an inch) in air, so that the discharges in the tube 
are caused partly by conduction from one terminal of the machine, and partly by 
induction due to the rapid alternations of high and low tension in the wire from 
the other terminal of the machine to the outside of the tube, the resulting discharge 
will be found to be sensitive. The arrangement here described, and which may 
be represented by the subjoined diagram, was devised by Mr. P. Ward, in order to 
show the phenomena of “ Crookes’ Lines.”'"' When the sparks passed to P, the outer 
surface of the glass near P was traversed, over an area of an inch or more in diameter, 
by irregular bright lines branching out from P as a centre. From the point of 
the inner surface adjacent to P, Crookes’ radiating lines shot out in various directions 
across the tube, and made themselves visible by fluorescence on the opposite side in the 
neighbourhood of II. The fluorescence figure, usually in the form of a cross, could be 
made to revolve about its centre by the action of a magnet ; the direction of the 
rotation being reversed when the polarity of the magnet was reversed. In this case 
the principal discharges in the tube take place when the passage of a spark from the 
one terminal of the machine to the other has caused a sudden relief of tension in the 
wires leading to the tube, so that the discharge has the same intermittence as 
the disruptive discharge between the terminals of the machine; in other words, it 
has the same intermittence as a long air-spark, and direct experiment with a revolving 
mirror has shown that the luminous phenomena in the tube opposite to the wire are 
synchronous with the passage of the sparks from one terminal of the machine to the 
other. 
Again, rapid intermittence and sensitiveness in what would otherwise be a 
continuous discharge may be produced by the use of a “ wheel-break,” such as is 
described (see Plate 15, fig. 3) in a paper iC On Stratified Discharges, III.” in the 
Proceedings of the Royal Society for February 15, 1877. This instrument consists of 
a wheel platinized at the edge, on which a platinum spring rests. In the circumference 
of the wheel a number of slots are cut, and filled with ebonite plugs, so as to interrupt 
the current. If the wheel-break be interposed in the circuit of a Holtz machine when 
producing a luminous discharge in a vacuum-tube, and the break be worked at a 
considerable speed so as to cause the current to be interrupted some 400 to 2000 
times per second, the discharge becomes highly sensitive. This instrument is used as 
a shunt, viz., so as to divert from the tube the current given by the machine by 
permitting it to pass through a continuous metallic circuit during the time that 
the platinum wire rests upon the metallic portion of the circumference. In tins way 
the current is never actually broken, and one great advantage of this arrangement is 
* See Proceedings of tlie Royal Society, December, 1878. 
