generated in a High-tension Magneto. 



165 



method, the secondary circuit including the large air- core 

 coil and the oil condenser. The oscillograph being used 

 idiostatically, the deflexion is proportional to the square 

 of the difference of potential, and each elevation in the 

 curve represents a half-oscillation. The first half-wave or 

 two represent the period during which the exciting spark 

 is passing, the remainder the free oscillation of the magneto 

 circuit. In fig. 7 the primary circuit was open and un- 

 connected with a condenser. In the case of fig. 8 this 

 circuit was closed. The curves illustrate the large 

 damping-effect of the core, which is much reduced when 

 the primary is closed owing to the fact that the core is 

 then partially shielded from the magnetic action of the 

 secondary current. 



6. Calculation of the Capacity of the Secondary Circuit. 



We are now in a position to form an estimate of 

 the value of the capacity of the secondary circuit of the 

 magneto 



L 2 = 19-3 . 10 9 c.g.s., 



Since 



L 2 C 2 = 1-48 . 10" 9 c.g.s. and 



we have 



C 2 = 0000077 microfarad. 



This estimate is, however, rather too low on account of the 

 fact that we have assumed too great a value for L 2 — 

 the value for uniformly distributed currents. Probably the 

 secondary capacity does not fall far short of 0*0001 micro- 

 farad, a value which is not exceeded by that of the secondary 

 of a very large induction-coil. It is about equal to the 

 capacity of a spherical condenser of radii 3 and 3'1 cm. 

 Large secondary capacity must be a feature of all H. T. 

 magnetos, owing to the fact that the coils are closely 

 surrounded by metallic surfaces at zero potential ; and 

 this fact must exercise a great influence, not only on the 

 spark-length, but also on the character of the spark and 

 the quantity of electricity discharged in it. 



In the case of an induction-coil^ when a condenser is 

 connected with the secondary terminals in parallel with 

 the gap, the discharge at moderate currents — currents which 

 are considerably greater than the minimum required to 

 produce the discharge — takes the form of a "multiple 

 spark," a large number of sparks sometimes passing (all 

 in the same direction) at each break of the primary 

 current. When the primary current is increased to a 

 certain value the discharge changes to the type usually 





