478 
DR. S. P. THOMPSON ON CATHODE RAYS 
toward an anodic point, whether that point be internal or external to the glass w^alL 
If a single “splash” is produced by a solitary discharge from the coil, the external 
surface of the tube at the part splashed shows an electrostatic state of charge 
evidenced by the adherence of dust.* The explanation of the phenomenon appears 
to be the following At this stage of exhaustion, the first portion of a cathodic 
dischaige electrifies the inner surface of the glass where it strikes, giving it a 
negative charge, or making it temporarily cathodic. The presence of this cathodic 
charge electrostatically affects the next-advancing portion of the discharge, and 
causes it to strike the glass a little on one side, so further distorting the ray. This 
ma^ occui m any direction from the central point and will obviously present an 
instability; the “splash” creeping outward from the centre, first in one direction, 
then in some other, ramifying as it spreads, and fading out almost as fast as it is 
formed. 
4. Cathode Shadows of Hot Wires. 
After the experiments described in § 2, in which threads of mercury in fine closed 
glass tubes were used to cast shadows, others were made to test the effects of 
electric currents passing in the object which casts the shadow. A tube wns 
constructed resembling fig. 11, but having the bulb traversed by a narrow glass 
tube which opened to the air at both ends. This was filled with mercury, and 
connected to a small battery to pass currents through it. No effect was noticed that 
depended either on the direction or the strength of the current. 
Another tube [No. C 4], (fig. 12), was made, having a thin bare platinum wm’e 
stretched across the bulb between inserted terminals, P, Q. This wire gave as its 
shadow in the cathode rays a fine black line, bordered by the usual margin of 
brighter luminescence. On sending through the wire a current from a small 
insulated battery of accumulators no effect was observed until the current had been 
so fai increased as to make the wire red hot, wdien its shadow was observed to be 
rather wider at the end by which the current left than at the end by which it 
entered. On reversing the direction of the current, this effect also changed direction. 
If, undei these circumstances, the wire was made anodic as a whole by connecting 
the insulated battery, or any part of its circuit, to the anode pole of the coil, the 
shadow of the wire at once changed to a luminous line (a negative shadow, in fact, 
as described in § 1 above), which showed no change on reversing the battery, and 
which was unaltered whether the current is on or off. On similarly making the 
wire cathodic, its shadow expanded to some 6 millims. wide; and again, no effect 
was perceptible on reversing the current, though, apparently, the wire wdien hot 
A similar observation has been made by Villari (‘Rendiconti della R. Accademia dci Liucei,’ 
■vol. o, May 1/, lb96), wlio Las investigated the external electrostatic state of Roxtgen tubes and 
Geissler tubes by the use of electroscopic powders. 
