1877.] Mr. W. Spottiswoode on Stratified Discharges. 



91 



the covered parts were brought face to face, a jar was formed the 

 size of which could be varied at pleasure. An air-spark of adjustable 

 length was also introduced into the circuit between the machine and the 

 tube. 



This arrangement was subsequently replaced by the following, which 

 in some respects proved more convenient : — A battery of one or more 

 jars was used in the place of the mica plates. The outside of this bat- 

 tery and one terminal of the tube were connected with the earth ; and 

 the inside and the other terminal were alternately connected with the 

 positive conductor of the machine, so that the battery was alternately 

 charged and discharged through the tube. The amount of charge was 

 regulated partly by the distance through which the conductors of the 

 machine were separated, and partly by the number of revolutions of the 

 machine during which the charging took place. It was consequently in- 

 dependent of the absolute time of contact. It will be observed that this 

 arrangement did not give the same opportunity of a continuous variation 

 of jar surface as the first ; but, on the other hand, the changes of phase 

 in the phenomenon due to increments of charge were capable of indefinite 

 diminution by shortening the distance between the conductors of the 

 machine and by increasing the number of the jars. 



The first object proposed was to ascertain whether a jar could be 

 charged with so small a quantity of electricity as of itself to give a 

 stratified discharge in a tube ; in other words, whether the resistance of 

 the tube itself, if resistance it be, could by a suitable charge of jar be 

 made to insure a stratified discharge. For this purpose a jar was charged 

 with small sparks from the machine, and discharged after receiving 

 charges of 1, 2, 3, &c. sparks in succession. The experiment proved 

 successful with a coal-gas tube at a pressure of about 4 millims. : charges 

 of three sparks gave bright flake-like stratifications ; higher charges gave 

 a discharge with a positive column, a negative glow, and a dark space, 

 although the stride were not always discernible. But when the charge 

 exceeded 5 or 6 sparks, the positive column advanced so far as to obli- 

 terate the dark space, and ultimately made its way to the hilt of the 

 terminal. 



Similar experiments were made with both forms of instrumental 

 arrangement, and with tubes containing different gases and at different 

 pressures. 



A number of tubes tried with various amounts of battery-charge, but 

 with the same surface, showed that, as the charge was increased, the head 

 of the positive column advanced towards the negative terminal, the dark 

 space became narrower, and the glow contracted in dimensions; and 

 when the head of the column drew very near to the negative terminal, 

 the glow, instead of covering the whole surface of the terminal, formed a 

 small drop at the point. On still further increasing the charge, the 

 drop withdrew to the hilt of the terminal ; and finally, when it had com- 



