17 



direct-current machine containing the 

 same amount of material, whilst the cur- 

 rent, owing to the concentration of wires, 

 and consequent greater liability to heat, 

 is something less than one-half. The 

 output of this machine will, therefore, be 

 something less than 40 per cent, greater 

 than that of a direct-current machine 

 having an equal weight. It must not be 

 forgotten that the results here obtained 

 refer to machines in which the active 

 wires on the armature are concentrated 

 into single lines. This is of course 

 impossible in practice, since the wires 

 must be of appreciable thickness, and 

 must therefore occupy a considerable 

 space. An approximation can be made 

 to the theoretical condition by employing 

 an iron-cored armature with projecting 

 teeth, the coils being laid into the re- 

 cesses. As, however, armatures of this 

 type tend to produce heating of the pole- 

 pieces, if the latter be solid, the arrange- 

 ment is not generally used ; and when 

 the armature is provided with an iron 

 core, the surface of this is in most ma- 



