572 



NATURE 



[April 13, 1893 



niagnetic forces acting upon them by their weight. Such a 

 locomotive is shown in the third model before you. So far as I 

 am aware, a locomotive of such simplicity as this has never been 

 constructed for practical work, but on the City and South 

 London line the armatures of the motors are placed directly on 

 the axles and the magnets, suspended partly from the axles and 

 partly from the frame. 



The second model is an exact reproduction of the locomotives 

 on the City and South London line, but with a different arrange- 

 ment of motors. Here both armatures are included in the same 

 magnetic circuit, and both magnets and armatures carried on the 

 frame of the locomotive and not on the axles. The armatures are 

 geared to the axles by diagonal connecting rods, the axle boxes 

 being inclined, so that their rise and fall in the horn blocks is at 

 right angles to the connecting rods. This design, which is due 

 to the late Mr. Lange, of Messrs. Beyer, Peacock & Co., 

 allows of the motor armature being placed on the floor level of 

 the locomotive, and so more easily accessible. 



This model will serve to show some of the characteristic 

 features as well as some of the characteristic defects of an 

 electric motor as 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 overcom- 

 ing 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 it^ 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 proper 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 im- 

 portance 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 locomo- 

 tives as a whole is 70 per cent, only, so that the loss in starting 

 is equal to 20 percent, 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-motive force balances the electro-motive force 

 of the line the speed can be increased by diminishing the 

 magnetic field by reducing the effective coils on the 

 magnets, but this device does not give any assistance at the 

 lower speeds, as the magnets ought to be so wound as to be 

 high on the characteristic curve, or nearly saturated, with the 

 normal current, and it is therefore not possible to obtain 

 any increased intensity of field, by increasing the convolutions 



NO. 1224, VOL. 47] 



of the magnet coils. If it were possible to use alternate current 

 motors for traction work the difficulty could at once be met by 

 introducing a transformer in the circuit, and placing the motor 

 in its secondary. The effective convolutions of the secondary 

 circuit on the transformer could then be varied as the speed 

 increases in such wise that the electro- motive force of the line 

 is balanced by the back electro-motive force of the motor and 

 the fall of potential due to the resistance of the motor coils, 

 so avoiding all need for resistances. 



The City and South London line has enabled experiments to be 

 made on the efficiency of the railway system as a whole, taking into 

 account the loss of power in the generators, on the line, and 

 in the motors, and in the resistances of the locomotives. The 

 loss in the line is about eleven per cent, of the power generated, 

 and the efficiency of the locomotives as a whole is, as I have 

 shown, 70 per cent, ; thus the electrical efficiency of the entire 

 system is 62 per cent. The trains weigh with full load of 100 

 passengers about forty tons, and the average speed between 

 stations is 13 5 miles per hour. The cost of working, including 

 all charges, during last half year was T\d. per train mile, of 

 which 47^, represents the cost of production of the electric 

 power, and 2'4(/. the cost of utilising it on the locomotives. It 

 is perhaps hardly a fair comparison to compare the cost of work- 

 ing such a line as the South London line with the cost of steam 

 traction on other lines, inasmuch as steam could not possibly be 

 used in the tunnels, only 10' 6" diameter, in which this line is 

 constructed, but the comparison is not uninstructive. Take the 

 Mersey Railway, where the gradients and nature of the traffic 

 are similar. On the Mersey Railway the locomotives weigh 

 about 70 tons, and the train, which is capable of carrying about 

 350 passengers, 150 tons. According to the published returns 

 of the company the cost of locomotive power is 14^. per train 

 mile, i.e. double the cost on the South London line, but for a 

 train weighing between four and five times as much but capable 

 of carrying only 3I times the number of passengers ; thus the 

 cost of steam traction per ton mile of train is about half that per 

 ton mile of train for electric traction. But it is not on the cost 

 per ton mile that the success of a passenger line depends. The 

 real basis of comparison is the cost per passenger mile, and here 

 electric traction has great advantage over steam, as the dead 

 weight of the electric motor is small compared with the dead 

 weight of steam locomotives of the same power, and with electric 

 motors the trains can be split up into smaller units at but slightly 

 increased cost, so permitting a more frequent service. We can- 

 not expect, therefore, that electric traction with our present 

 knowledge will take tbe place of steam traction on our trunk 

 lines ; but it has its proper function in the working of the under- 

 ground lines now projected for London, Paris, Berlin, and 

 Brussels, and other large towns, and also I think on other urban 

 lines, for example, on the Liverpool Overhead Railway, where 

 trains of large carrying capacity are not required, but a frequent 

 service is essential ; and finally, also on those short lines, whether 

 independent or branches of the great trunk lines where water 

 power is available. When I undertook the construction of the 

 Bessbrook line it was a condition that the cost of working should 

 be less than the cost of working by steam, a condition which 

 the fiist six months of working showed to be successfully ful- 

 filled. When Messrs. Mather and Piatt undertook the con- 

 struction of the electric plant for the City and South London 

 Railway, they guaranteed that the cost of traction for a service 

 of 8247 miles (ler week as actually run should not exceed 6 ■3a'. 

 per train mile, exclusive of the drivers' wages. Their anticipa- 

 tions have been more than realised, the actual cost being 

 ^•id. per train mile only. There are, however, other projects 

 both in America and on the continent for electric railways on 

 which the special feature is to be an enormously high speed of 

 travel, speeds of 150 and even 200 miles per hour being prornised. 

 With a steam locomotive, involving the reciprocating motion of 

 the piston and connecting rod, such speeds are probably un- 

 attainable, but they may be realised in the purely rotary motion 

 of an electric motor. But at such high speeds as these the 

 power required to overcome the air resistance is of special con- 

 sideration. Probably up to speeds of 750 miles per hour, or 

 even to higher limits still, the ordinary law of air resistance holds 

 good, as the rate of disturbance is still less than the velocity of 

 waves in air, but above these limits we leave the regions of 

 ordinary locomotion and enter rather into the field of projectiles. 

 Assuming, however, that the ordinary laws of air resistance do 

 hold good, I calculate that the power required to propel an 

 ordinary train 200 feet long at 200 miles per hour against the 



