Resistance , and Self-Capacity of Magneto Windings. 63 



winding only, all the electro magnetic energy is dissipated 

 in iron, copper, and stray losses. The efficiency of con- 

 version is zero. Suppose the secondary to have the same 

 number of turns as the primary. The secondary peak 

 voltage and the self-capacity are extremely small, and there- 

 fore the electrostatic energy represented by the expression 

 ^CyV^ 2 is very small. As the number of secoudary turns is 

 increased, the electrostatic energy is also increased (apart 

 from losses) and with it the efficiency of conversion, until it 

 reaches a maximum value. With a given number of primary 

 and secondary tarns and a fixed primary condenser, Taylor- 

 Jones has shown that for a certain induction-coil the peak 

 voltage and, therefore, the electrostatic energy prior to the 

 passage of the spark has an optimum value, when the coupling- 

 is about 0'57. This is due to the phase relationships of 

 the two sets of oscillations, i. e., their combined effect in 

 enhancing the peak potential is a maximum for this degree 

 of coupling. In the magneto it is not practicable to vary 

 the coupling by withdrawing the primary core, but it can 

 be varied by adding external air-core inductance to the 

 secondary. Experiments by Morgan * show that the peak 

 voltage can be increased in this manner. 



If at the moment at which the spark commenced, the 

 energy of the primary was zero, the electrostatic energy 

 obtained from the magneto per se would be the maximum 

 possible. By reducing the losses this maximum would 

 increase until finally the electromagnetic energy in the 

 primary would be wholly converted into electrostatic energy 

 in the secondary. 



There is another method of increasing the secondary 

 electrostatic energy apart from that of altering the coupling. 

 If a condenser is connected in parallel with the secondary, 

 the product iC2V2 2 increases with the capacity up to a 

 certain point. For example, with a total capacity of 

 600 picofarads the peak voltage of a magneto was reduced 

 from 10,000 to 5,000 at* a certain speed. The self-capacity 

 of the magneto was 65 picofarads, so that the energy was 

 increased 150 per cent. Unfortunately, however, the peak 

 voltage, which appears to be the prime factor in ignition, 

 was decreased 50 per cent. Since the primary current 

 was the same in both cases, it is clear that the conversion 

 efficiency was also increased in the same proportion as the 

 energy. 



Returning to the example cited in section (7), the initial 

 energy in the primary winding was not taken into account. 



* Morgan, ' Electric Spark Ignition' (Crosby Lockwood). 



