146 SCIENCE PROGRESS. 



they are displaced ahead of, or behind, the line of symmetry, 

 VS, between the poles, the convolutions or loops included 

 within the arcs mn and op have a direct magnetising effect, 

 which will either increase or decrease the total number of 

 lines passing through the armature according as the current- 

 sheets embraced between the points m, 11 and 0, p flow in 

 the one or the other direction. The remaining convolutions 

 included within the arcs mo, np were shown to produce "a 

 material disturbance in the distribution of the induction over 

 the bored face of the pole-piece " : if the edge of a pole by 

 which an inductor first enters into its field be called the 

 "leading" pole-tip as opposed to the "trailing" tip by 

 which it leaves the pole, then the lines are driven away 

 from the leading corners // and are crowded up towards the 

 trailing corners tt. As, however, the weakening of the 

 field at the leading pole-tip is counterbalanced by the 

 strengthening of the field at the trailing pole-tips, 

 "this disturbance has no material effect upon the per- 

 formance of the machine". In the ordinary dynamo the 

 brushes are advanced ahead of the line of symmetry 

 in order to avoid sparking, i.e., assuming the direction 

 of rotation shown in fig. 2, to a position corresponding 

 with the line nip. In this case, if the poles be as 

 shown in the diagram, the current in the wires m to n 

 is directed towards the observer as indicated by the 

 dotted circles, and away from the observer in the cor- 

 responding wires to op as shown by the crossed circles. 

 On comparing their directions with the direction of 

 the current in the neighbouring wires of the magnet 

 winding proper, it will be seen that they are exactly 

 opposed to each other : in other words, the magnetising 

 effect of the loops between mn and op is opposed to the 

 magnetising force of the fixed coils on the magnet. They 

 are, in fact, "back ampere-turns" neutralising an equivalent 

 number of the forward ampere-turns of the stationary mag- 

 net-coils. On the other hand, the loops between mo and np 

 are " cross ampere-turns " distorting the field by twisting 

 it round in the direction of rotation. This division of the 

 armature ampere-turns into two sets was brought into 



