1877.] Mr. W. Spottiswoode on Stratified Discharges. 549 



This is, ill fact, identical with the suggestion there made, that the flutter- 

 ing appearance was due to the unequal duration of the striae themselves, 

 and to the iiTegular positions of the points at which they were renewed 

 at successive discharges of the coil. And such, in fact, proved to be the 

 case ; for as the speed of the wheel was increased the duration of the 

 discharges diminished ; the image as seen in the mirror became narrower 

 and simpler in its configuration, until, when the steady effect was pro- 

 duced, each discharge showed only a single column of striae of a width 

 proportional to the apparent width of the slit. The proper motion, 

 implied by the inclination of the individual striae to the vertical, w^as still 

 perceptible, and was directed, as usual, towards the negative pole. 



Trom a comparison of the number of striae as seen by the eye with 

 those seen in the revolving mirror, it was found that the striae so formed 

 were of the kind called " simple" in former communications. And the 

 phenomena of the flow may therefore be considered to be due to the different 

 positions taken up by the striae in successive discharges. If in each dis- 

 charge the striae occupy positions in advance of those occupied in a 

 previous discharge, the column will appear to advance ; if the reverse be 

 the case, they will appear to recede. If the positions remain unchanged, 

 the column will appear stationary. 



The following consequence of this explanation of the flow will readily 

 occur to the reader, viz. that the rapidity of the flow will increase with 

 the extent of advance made by the striae in each successive discharge, 

 until that advance amounts to half the distance between two contiguous 

 striae. Before this is attained the flow vAVl have become too rapid to be 

 followed by the unassisted eye, and can only be observed by the aid of 

 the mirror. When this rate of advance has been exceeded, the flow will 

 appear to be reversed. If the rate of advance still continues to increase, 

 the rapidity in the reverse direction will appear to decrease until the 

 advance amounts to the entire distance between two contiguous striae, 

 when it will apparently be reduced to zero ; the striae will then again 

 appear stationary. Experiments appear to confirm this view of the case. 



Experiments were next instituted with a view of ascertaining the con- 

 nexion between the flow and resistance. Starting from a condition of 

 current and break for which the striae were stationary, it was found that 

 an increase of resistance, introduced generally in the primary circuity 

 produced a forward flow, i. e. from the positive towards the negative 

 terminal, while under similar circumstances a decrease of resistance pro- 

 duced a backward flow. Eurthermore, if after producing a forward flow 

 the resistance be continually increased, the flow after increasing in 

 rapidity so as to become indistinguishable by the unassisted eye, gradually 

 appears to become slower, and ultimately to reverse itself, in accordance 

 with the law suggested above. 



Another form of contact-breaker was also occasionally used. The 

 principle upon which it was based was the sudden disruption of a thin 



2 E 2 



