CONTINUOUS-CURRENT WINDINGS 137 



with the subject on quite the same lines as would be suitable in a 

 treatise intended to be employed exclusively by those entrusted 

 with the task of the pre-calculation of the electro-magnetic design. 

 Those so engaged can profitably familiarise themselves with the 

 aspects of the subject on which emphasis is laid in this treatise ; 

 they should, however, also master the further aspects of the subject 

 as set forth in treatises dealing more especially with electro- 

 magnetic design. 1 



It is believed that, in view of the above explanation, the less 

 frequent allusion to the question of current distribution in the 

 windings will be appreciated. Briefly, it may be stated that where 

 desirable to indicate the paths of the current, the direction through 

 the winding from negative to positive brushes is indicated by 

 arrow-heads affixed to the radial lines representing face conductors. 

 As the current changes its direction as -the conductors pass under 

 each successive pole, it is only practicable in a single diagram to 

 indicate the direction at some particular instant. The rule for 

 ascertaining the direction of the current for the case of a machine 

 when employed as a generator, 2 is as follows : 



When a conductor passes under a north pole in a clockwise 

 direction from the observer's standpoint, the electro-motive force 



1 Amongst these treatises, the writer naturally prefers those in which the 

 nomenclature here employed has been adopted. The most extensive treatise 

 is Armature Windings, Parshall & Hobart (New York, D. van Nostrand Co., 

 1895). 



The subject is much more briefly dealt with in Electric Machine Design, 

 Parshall & Hobart (London, offices of Engineering, 1906) ; Electric Motors, 

 H. M. Hobart (London, Whittaker & Co., 1904) ; and Elementary Principles of 

 Continuous-Current Dynamo Design, H. M. Hobart (London, Whittaker & Co., 

 1906). 



Dr Thompson and Prof. Arnold have handled the subject most ably ; the 

 nomenclature they employ is, however, considerably at variance with that in 

 the other treatises and in the present volume. W. Cramp, however, in a most 

 interesting little volume, Armature Windings of the Closed Circuit Type (London, 

 Biggs & Co., 1906), has employed substantially the nomenclature adopted in the 

 present treatise. 



2 It is almost superfluous to state that when the machine is run as a motor, 

 the current will flow in just the opposite direction for a given direction of 

 motion of the conductor under a pole of a given polarity. The electro-motive 

 force induced in a conductor by its passage through the magnetic field, is the 

 same in both cases ; but in the case of a generator, the current flows under the 

 influence and in the direction of this induced electro-motive force, whereas in 

 the case of a motor, it flows in the direction and under the influence of an 

 external electro-motive force which is greater than the induced (and, in this 

 case, counter) electro-motive force in opposition to which it flows. 



