76 ELECTEIC TBACTION. 



shunt and series winding for the purpose of enabling the 

 machine to do either a large or small amount of work. Many 

 other Americans produced dynamos, amongst them being 

 Edison. The dynamo was now brought up to a great state 

 of perfection, and since the year 1&83 the chief progress 

 made has been in details of design and mechanical construc- 

 tion. 



I will now proceed, as briefly as possible, to illustrate the 

 physical theory of dynamo-electric machines, which I hope 

 will enable us to understand more clearly the part they play in 

 the propulsion of tramway cars. Professor Sylvanus Thompson 

 defines a dynamo- electric machine as " a machine for convert- 

 ing energy in the form of mechanical power into energy in 

 the form of electric currents, or vice versa, by the operation 

 of setting conductors to rotate in a magnetic field, or by 

 varying a magnetic field in the presence of conductors/' 

 Projected on the screen is a magnet with electric tufts from 

 the North to South poles. (Plate I.) If iron filings were placed in 

 these magnetic fields, they would arrange themselves thus : — 

 (Plate II.) Now, if we place a conductor, such as apiece of copper 

 wire, in this magnetic field, as it is technically called, and moved 

 it about intercepting these lines, an amount of electricity 

 would be generated in the conductor at right angles to the 

 course of the movement of the conductor, and also at right 

 angles to the direction of the lines of force. " This induced 

 current is purely conditional on the intensity of the magnetic 

 field, and the length and velocity of the moving conductors, 

 as well as the resistance of the wire to the current." Now, 

 as to what this current is, no one knows. It is one of those 

 ultimate scientific facts at present behind the veil. Here is 

 another diagram showing the magnetic field surrounding 

 the conducting wire, end on. (Plate III.) Every 

 wire, as Professor Thompson puts it, " is surrounded 

 by a sort of magnetic whirl," thus: — (Plate IV.) 

 To do this wants energy, and that constant, if to be main- 

 tained. It is these whirls which act on magnets, and cause 

 them to set, as galvanometer needles do, at right angles to 

 the conducting wire. It must, however, be remembered that 

 a moving conductor in its motion must cut the lines of force 

 that pass through the circuit of which the moving conductor 

 forms part. I cannot illustrate this fundamental truth better 

 than by throwing upon the screen a few diagrams of Pro- 

 fessor Thompson's, and by quoting him on the subject; — 

 " If a coil or wire circuit be moved along in a 

 uniform magnetic field, as indicated in this diagram 

 (Plate V.), so that only the same lines of force 

 pass through it, no electric current will be gene- 

 rated ; or if, again, as in this diagram (Plate VI.), the coil be 



