54 Dr. McLachlan on Effective Inductance, Effective 



obtained using a definite current, i. e. the variation in H was 

 nearly constant. The reduction in the hysteresis and eddy- 

 current loss results in a diminution in the effective resistance. 

 The decreased variation in the flux density causes a diminution 

 in the inductance. The same 'effects were observed in con- 

 nexion with the secondary winding. Owing to the fact that 

 the iron forms a greater proportion of the mean cross-sectional 

 area of the primary than of the secondary, the polarization 

 effect was more marked in the first case than in the second. 

 Just before advance break the polarization is nearly zero and 

 the influence of the magnet should, therefore, be zero. This 

 was found to be true, since any variation in inductance was 

 within the limits of experimental error. The points obtained 

 are shown in fig. 10 thus : Q. It is evident that they follow 

 nearly the same curve as those obtained with the magnet 

 removed. The slight difference is due to the slightlv altered 

 configuration of the magnetic circuit before and after break. 

 In the absence of polarization, the variation of inductance 

 with the position of the rotor is considerably reduced. In 

 order to determine the influence of the metal of the magnet, 

 apart from polarization, a spare magnet was demagnetized 

 and fitted to the magneto. There was an increase in the 

 secondary inductance, when measured after advance break, 

 of about 5 per cent. 



(d) Effect of Armature Brass End-Plates. 



The brass end-plates are fixed to the core as shown in fig. 15 

 by bending two iron plates, AA ; at each end and riveting- 

 over to the end-plates, B B. Considering each end to be 

 replaced by a single turn of thin wire of radius a, it is clear 

 that no current would flow in the system, since theE.M.F.'s 

 in the two coils at_ any instant would be equal and opposite. 

 Owing, however, to the fact that there are large masses of 

 metal abo^ e and on each side of the slots in the end-piates,. 

 eddy currents are induced therein and the circuits of these 

 currents act as tertiary circuits when the magneto is in 

 operation, and also when undamped oscillations are passed 

 through the primary or the secondary winding. The effect 

 of the end-plates can be studied by means of an endless coil 

 of copper wire having several turns of 18 s.w.g. The end- 

 plates are removed ; the primary of the armature with core 

 is taken from its housing and connected with the circuit of 

 fig. 2. The circuit is brought to resonance, and the coil is 

 then slipped into the position occupied by one of the end- 

 plates. It is found that the effective external inductance 



