5J:8 Mr. W. Spottiswoode on Stratified Discharges. [Feb. 15; 



tried ; but the simplest, and ou the whole the best, proved to be dilute 

 sulphuric acid, in the proportion of 1 drop of acid to 6 drams of 

 water. Generally speaking the better conductor the fluid is, the better 

 are the results obtained ; but, o^ing to the insulating slots being very 

 narrow in this instance, a comparatively weak mixture of acid and water 

 was necessary. In one wheel, where the insulating slots are | in. wide, 

 a mixture 36 times as strong may as advantageously be used. The 

 spring, which under these circumstances became unnecessary, was replaced 

 by a point, the adjustment of whose distance from the wheel was simpler 

 and more accui-ate. This arrangement gave excellent results, even when 

 the number of currents per second was reduced in some cases to 250 ; 

 added to which the unpleasant and disturbing noise of the friction was 

 entirely avoided. 



"\Mieels having different numbers of teeth were also constructed, and 

 (what was perhaps of more importance) having teeth of different breadths, 

 so as to give with the same velocity of rotation contacts of different 

 duration. The breadth of the ebonite plugs, or length of interruption of 

 the ciu-rent, was immaterial, so long as the current was efficiently broken. 

 It did not appear, however, that with the same tube more could be 

 obtained vdth wheels having different nmnbers of teeth, than with the 

 same wheel at different speeds. But it was found that for different tubes 

 different wheels occasionally gave better results. 



A\lth the contact-breaker here described effects similar to those pro- 

 duced by the rapidly vibrating break were obtained. The striaB were 

 formed in a regular manner, and advanced or receded, or remained at 

 rest, in a column usually unbroken, so long as the velocity of the wheel 

 was maintained without change ; and even in the longer tubes, where the 

 striae, of the double discharge, advaiiced or receded towards both ends at 

 the same time, and appeared sometimes compressed and at others dilated, 

 the phenomena alwrys maintained their characteristic features. 



The condition of the striae here described, whether flo^dng or stationary, 

 may be comprised under the general term " steady;" and when there is 

 no motion in either direction, they may be specifically denominated as 

 *' stationary." 



Two questions here presented themselves : — First, what are the con- 

 dirions necessary for the production of steady striae ? Secondly, what are 

 the conditions and circumstances of the advance and retreat, in other 

 words, of the flow of steady striae ? 



With a view of ascertaining the nature of the distinction between the 

 ordinary and the steady striae, careful observations were made with the 

 revohing mirror. It having been noticed that when the wheel break 

 moved slowly ordinary or irregular striae were produced, and that when 

 it moved rapidly steady striae resulted, it seemed probable that the latter 

 effect might be due to the short time of contact, and to the consequent 

 absence uf inauy of the features described in Part II. of these researches. 



