CH. XII AEMATUKE REACTION 271 



to pass in the armature. The magnetisation curve was 

 then obtained by using exploring brushes in the way 

 already described. As the armature is series-connected 

 with two brushes the exploring brushes could be passed 

 freely round the surface of the commutator crossing 

 the positions virtually, though not actually, occupied 

 by a brush. In the figures, A and B represent the 

 positions thus virtually occupied by brushes ; when brush 

 A or brush B is mentioned we shall refer to the brush 

 diametrically opposite to the positions A and B. 



The magnet current was then broken and a current of 

 1 2 amperes passed through the armature ; the armature 

 was rotated as before, and the curve of armature reaction 

 obtained. The same currents were then passed in magnets 

 and armature at the same time, the armature was rotated 

 as a generator in the direction shown by the arrows, and 

 the resultant curve obtained. Each of these experiments 

 was repeated with the full load of 24 amperes, and with 

 36 amperes, half overload. 



The width of the brushes is given by the fact that they 

 cover two commutator bars and two insulations, or an 

 angular breadth of six degrees. Fig. 66 shows that they 

 were not well set, and this is borne out by the form of the 

 magnetisation curves. In all cases when the magneti- 

 sation due to the magnets only was being observed, the 

 brushes were removed from their holders. 



There was no sparking at all in any of the 12 ampere 

 experiments, although there was a field under both the 

 brushes of the wrong sign for commutating, particularly 

 under brush B, where the field due to the magnets is also 

 of the wrong sign. 



With the full load of 24 amperes, there was no spark- 



