as a Generator and as a Motor. 23 



(2) In both generators and motors " lead," distortion, or 

 displacement of brushes or of magnetic field is wrong, and is 

 to be avoided by attention to (1). 



Whatever " lead " there may be in either case, there is this 

 difference, that in dynamos this " lead" is in the direction of 

 rotation ; in motors it is in the opposite direction, as the course 

 of the current through the armature is reversed, but the field 

 is the same. 



(3) In both generators and motors absence of sparking at 

 the brushes depends mainly on the conditions of (1) being 

 complied with. 



(4) Reversal of rotation. In neither generators nor motors 

 is movement of the brushes necessary. 



But having got so far, a little consideration suggested the 

 probable existence of another analogy. Since a dynamo 

 having the above theoretically perfect form and action, with a 

 constant field would produce a constant electromotive force if 

 run at a constant speed, independently of the load or amount 

 of current generated, a motor constructed on the same prin- 

 ciples and having a constant field, if supplied with energy at 

 a constant difference of potential, should run at a constant 

 speed, independently of load. 



If this should prove to be a true analogy, a simple means 

 of obtaining results of great use in the practical applica- 

 tion of electricity would be obtained — results which have been 

 sought in many directions, more or less complicated, without 

 any great success. 



By permission of the Anglo-American Brush Electric Light 

 Corporation experiments were carried out in the testing-room 

 of their works, with a " Victoria " dynamo. The results ulti- 

 mately obtained, which are given in the following tabulated 

 account of the experiments, and in the curves plotted for them, 

 show that this fifth analogy is as true as the preceding ones, a 

 constancy of speed being obtained which was very remarkable 

 — even when the load was increased, until much more than that 

 which as a generator was usually considered the full working 

 current was traversing the armature. 



Two sets of readings were taken, working up to about th« 

 same current in each set, but with the potential-difference of 

 the supply different in the two cases, as stated. The field was of 

 the same strength throughout. The load consisted of another 

 " Victoria " dynamo driven through a modified White's trans- 

 mission-dynamometer, the work being varied as required by 

 altering the external circuit of this dynamo. 



