54 Mr. Edward Hopkinson [Feb. 24, 



such. But in order to show these clearly I may refer for a moment 

 to the general theory of a motor. It is easily shown that in a series 

 wound motor the couple or turning moment on the axle is a function 

 of the current only, and independent of the speed and electro-motive 

 force. Again, it follows from Ohm's law that the current passing 

 through the motor multiplied by the resistance of the magnet and 

 armature coils is equal to the difference between the electro-motive 

 force at the terminals of the motor and the electro-motive force which 

 would be generated by the motor, if it were working at the same 

 speed as a generator of electricity, that is to say the difference between 

 the electro-motive force at the terminals and what is called the " back " 

 or " counter " electro-motive force of the motor. Hence if the 

 terminals of the motor be coupled direct to the line at the moment of 

 starting when the motor is still at rest, the current will be very great 

 and its power entirely absorbed in the coils of the armature and 

 magnets, but the turning moment will then be a maximum. The 

 motor then begins to move, part of the power being spent in over- 

 coming frictional resistances and part in accelerating the train. A 

 back electro-motive force is then set up, increasing as the speed 

 increases, and causing the current to diminish until finally a position 

 of equilibrium is established, when the speed is such that the back 

 electro-motive force together with the loss of potential in the coils of 

 the motor is equal to the potential of the line. But in practice the 

 mechanical strength of the motor, and the carrying power of its coils, 

 as well as the limited current available from the generators, makes it 

 necessary to introduce resistances in circuit with the motor to throttle 

 the current and to reduce it within projDer limits. It is to this point I 

 desire to draw attention, that in traction work when starting the 

 motor resistances must be introduced, which, with the resistance 

 of the motor itself, at the moment of starting, absorb the whole 

 power of the current, reducing the efficiency of the motor to nil, 

 and which continue to absorb a large percentage of the power, until 

 the condition of equilibrium is established. This is the great 

 defect in electric motors for traction work, and its importance can be 

 shown very clearly by reference to the work done on the City and 

 South London line. There the motors when working with their 

 normal current have an efficiency of 90 per cent., but the actual all- 

 round efficiency of the locomotive as a whole is 70 per cent, only, so 

 that the loss in starting is equal to 20 per cent, of the whole power. 

 Of course in some respects the City and South London line is 

 exceptional in that a start is made every two or three minutes. 

 Various devices have been suggested with a view to diminishing this 

 waste of power in starting an electric motor, but none entirely meet 

 the case. Thus, if the locomotive or car has two motors, these can 

 be coupled in series at the start, and subsequently thrown into parallel, 

 thereby doubling the tractive force with a given current, or for the 

 same tractive force reducing the loss of power by three-fourths. 

 When through the increase of speed of the motor the back electro- 



