36 Dr. McLachlan on Effective Inductance, Effective 



removed from the housing. It is found by experiment that 

 the above conclusion is correct. (Pee fig. 10, curve 3.) 



It is clear that the air-gaps cause the effective permeability 

 to attain an approximately constant value at ;i much lower 

 frequency than would otherwise be the case. Moreover, the 

 primary and secondary inductances will diminish with the 

 frequency until they are nearly constant. This is an un- 

 desirable feature for high peak-voltage in the magneto. 

 Hence the air-gaps should be reduced to a minimum, and 

 the apparent permeability of the iron should — at the higher 

 frequencies — be as large as possible. This latter condition 

 can be fulfilled by using very fine laminations — i. e. as thin 

 as conditions of economical manufacture will allow. It is 

 also necessary that the brand of iron used should have a low 

 remanence, sm all hysteresis, and high resistivity to ensure 

 small loss. There is one point to be noted, however, in 

 connexion with fine laminations. It can be shown* that 

 with a given brand of iron, for a given total cross-sectional 

 area of the magnetic circuity e. iron + insulation between 

 sheets, there is a certain thickness which gives optimum 

 peak-voltage, and, therefore, maximum secondary energy 



It is w T ell known that at frequencies of the order of 50 ~ 

 per sec. the primary inductance of a magneto depends on the 

 position of the armature with reference to th^ pole-shoes. 

 This can be accounted for in two ways : (1) the variation in 

 length of the iron and air-gaps completing the magnetic- 

 circuit through the armature core, i. e. the variation in 

 reluctance ; (2) the variation in polarization of the iron due 

 to the flux from the magnet. An increase in either the 

 reluctance of the magnetic circuit of the armature or the 

 polarization, entails a decrease in the inductance. This also- 

 applies to the secondary inductance, since both primary and 

 • secondary are wound on the same core. In the inductor type 

 magneto, the polarization and reluctance have minimum values 

 just before break, when the tuning lever is set at full advance. 

 In this position the greater part of the magnetic circuit 

 through the primary is unpolarized, since th^ flux through 

 the primary winding is zero, and the circuit is completed 

 across the inductor. We should expect, therefore, the in- 

 ductance to be a maximum in this position, and this is 

 confirmed by experiment. In any other position of the 

 timing lever, the magnetic circuit through the primary 

 winding is completed partially, at least, through the magnet, 

 and the reluctance of the circuit is greater owing to the 

 * See McLachlan, < Wireless Year Book,' 1918, p. 898. 



