166 Prof. E. Taylor Jones on the Potential 



found when there is no secondary condenser — a single 

 spark followed by an arc. 



In the magneto also the discharge at moderate currents 

 may take the form of a multiple spark (see Plate VII. 

 fig. 10), doubtless owing to the large capacity of its 

 secondary coil. 



Another effect of connecting a condenser with the 

 secondary terminals of an induction-coil is to cause, for 

 a given small or moderate current and a given spark-gap, 

 a large diminution in the quantity of electricity discharged 

 in the spark. 



In some ways the large capacity of the magneto may 

 act beneficially : for example, by increasing the periods of 

 oscillation, and thus lengthening the duration of the high 

 potential, it enables the spark to appear more readily ; 

 again, a large secondary capacity involves a large optimum 

 primary capacity, which is an advantage, since the inter- 

 rupter works better when associated with a condenser of large 

 capacity. It should also be remembered that, other things 

 being the same, an increase in the secondary capacity does 

 not necessarily cause a diminution of the secondary potential 

 for a given primary current; from the point of view of 

 secondary potential there is an optimum secondary as well as 

 primary capacity. It is also said that condenser-discharge 

 sparks are specially favourable to the production of ignition. 



7. Oscillations daring the Discharge. 



Some photographs were taken of the magneto spark by 

 focussing the image on a sensitive plate with a rotating 

 concave mirror. Two of these are shown in Plate VII. 

 figs. 9 and 10. 



The photograph in fig. 9 was taken at the interruption of 

 a primary current of 1*8 amperes. It shows an initial spark 

 followed by an arc on which are superposed a number of 

 fine regularly-spaced bands representing small oscillations. 

 Six or eight of these bands are visible on the negative, and 

 though faint they could be measured with fair accuracy 

 under a low-power microscope. Their frequency is about 

 15,800 per second. These are the oscillations of the system 

 with the secondary closed by the arc, and their period is, 

 by equation (15) with C = cc, given by the expression 



^\/ Li0i(1 r F) , 



where k 2 is the coupling for uniformly distributed currents 

 of the mean value used in this experiment. 



