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CHAPTER XII 



ARMATUEE REACTION 



THE magnetisation curves of a dynamo can be found by 

 substituting for the ordinary brushes a pair of thin steel 

 brushes insulated from one another and touching the 

 commutator at a small angular distance apart. If these 

 brushes are connected to the terminals of a voltmeter, and 

 the armature rotated at a uniform speed when the magnets 

 are excited, the reading on the voltmeter will measure 

 the rate at which the conductors on the armature, included 

 between the two exploring brushes, are cutting lines of 

 force. If the brush holder is moved round the commu- 

 tator, the voltmeter reading will vary with the intensity of 

 the magnetisation measured across the surface in which 

 the conductors on the armature are moving. 



If the readings observed are plotted vertically on a 

 line along which distances represent successive positions 

 of the exploring brushes, we shall obtain a curve whose 

 ordinates represent the intensity of magnetisation round 

 the armature. The curve in Fig. 64 marked ' magnets 

 only ' is such a curve. The magnetisation is nothing 

 at points a and fr, and is uniform under the poles, the 

 sign of the curve being different under adjacent poles. 

 The actual values of the voltmeter readings will depend 

 upon the angular width of the exploring brushes ; hence 



s 2 



