300 Mr. W. Duddell on a 



whole clamped together on a tool-steel spindle. The in- 

 ductor alone weighed about 200 grammes. As at first con- 

 structed, the edge of the inductor had 30 V-shaped notches 

 cut in it so as to have 30 flat-topped teeth. 



Surrounding the inductor is a laminated soft-iron ring 

 having two inwardly projecting poles; the clearance between 

 these and the teeth of the inductor, which formed the air-gap 

 of the machine, being less than 0*1 mm. The ring itself is 

 wound so that a direct current flowing round the winding 

 tends to produce lines of force from one pole to the other 

 through the inductor. The number cf lines of force varies 

 from a maximum when the teeth on the inductor are opposite 

 the poles to a minimum when the poles are in front of spaces. 

 This variation in the total number of lines of force produces 

 alternating E.M.F/s in any winding either on the pole-pieces 

 or on the ring itself ; the movement of the inductor through the 

 distance between two consecutive teeth producing one complete 

 period. One winding on the ring could serve to carry both the 

 direct field-current which magnetizes the ring and the alter- 

 nating current produced. In practice, owing to the great im- 

 portance of any self-induction in the alternate-current circuit, 

 it was found advisable at high frequencies to use separate 

 windings for the two currents. The ring was, therefore, wound 

 in three sections as follows : — Two sections, each consisting of a 

 single layer of 113 turns of No. 33 double cotton-covered wire 

 wound next the core on each side of the ring which could be 

 connected in series or parallel, were used as the armature. 

 The third section consisting of 4 layers wound on top of the 

 above, having 430 turns on each side of the ring, the whole 

 being permanently connected in series, was used as the field- 

 coil. At a later date coils on the pole-tips themselves were 

 added. 



In whatever way the windings are arranged or used, it is 

 necessary to prevent the induced E.M.F/s from sending 

 alternate currents round the exciting circuit, outside the 

 alternator, as these currents would tend to annul the changes 

 in lines of force through the coils and so prevent any useful 

 alternating current from being produced. To prevent this 

 the exciting circuit contained a choking-coil. 



As no steam turbine or other high-speed motor was 

 available, the alternator was at first driven by means of a 

 figure-of-8 belt-drive, illustrated in fig. 3, from a J H.P. 

 direct- current motor D fixed, with its shaft vertical, to a 

 heavy wooden framework. Two bicycle-wheels 24J inches 

 actual diameter, No. 14 tangent-spokes, and " Palmer's " 

 jointless hollow rims, w 7 ere employed : the one fixed direct to 



