42 



NATURE 



{May 9, 1889 



•seen in Fig. 5. A small pinion rotated by the dynamo 

 shaft rocks a double ratchet by means of an eccentric. 

 One or other of the ends of this ratchet is pulled down 

 and made to engage with the ratchet wheel depending 

 on whether the current is greater or less than 50 amperes, 

 the normal current supplied to the line. The rotation of 

 the ratchet wheel alters the positions of the brushes on 

 the commutator, until the current is diminished or in- 

 creased to 50 amperes, when the iron core attached to 

 the ratchet, and which was previously sucked down below 

 its normal position by the solenoid, or not sucked down 

 so far, is now held in its normal position, and the ratchet 

 kept free of the ratchet wheel. This mechanical con- 

 stant current regulator works well, with a surprisingly 

 small amount of sparking at the commutator. 



As constant current is supplied to the line, the speed 

 ■of the car could not be altered by introducing resistance 

 into the circuit ; what is done therefore is to shunt the 

 field magnets of the motor with a less and less resistance 

 by moving a lever on the car, as less and less power is 

 required to be developed by the motor. When the car 

 is at rest at the end of the journey, the motor is entirely 

 cut out of the circuit by the handle being pushed full 

 home. With horse tramcars a mechanical brake must 

 be employed, and the energy of the moving car wasted in 

 friction ; indeed with the continuous vacuum brakes on 

 the modern railway trains, not merely is the energy of the 

 train wasted, but coal is actually burnt to stop the train. 

 Some years ago, however, Profs. Ayrton and Perry pointed 

 ■out that when an electric train was running down hill, or 

 when it was desired to stop the train, there was no 

 necessity to apply a mechanical brake and waste the 

 energy of the moving train in friction, because by turning 

 a handle the electromotor could be converted into a 

 dynamo, and the train could be slowed or stopped by its 

 •energy being given up to all the other trains running on 

 the same railway, so that trains going down hill could 

 help the trains going up hill, and the stopping trains 

 could help the starting trains. And at. that time they 

 proposed detailed methods for carrying out this econo- 

 mical mutual aid arrangement whether the trains were 

 running in parallel or in series. But there is this great 

 difference between the two systems, that whereas with 

 motors in parallel it is only as long as a stopping train is 

 still running fairly fast that it can help other trains ; on 

 the other hand, when one of a group of motors in series 

 has been temporarily converted into a dynamo by the 

 reversal of the connection of its field-magnets, this motor 

 can return energy to the system down to the very last 

 rotation of its armature. This difference, which is greatly 

 to the advantage of the series system, will be easily 

 understood if it be remembered that a motor will help 

 other motors in series with it if it supplies a forward 

 ■electromotive force, however small, whereas if the motors 

 be in parallel it is necessary, in order that a motor should 

 give energy to the system, that it should be able to 

 reverse the direction of the current that was previously 

 passing through it — in other words, produce an electro- 

 motive force actually larger than the potential difference 

 set up between the mains by the dynamo itself. • 



And not only is this form of brake very economical in 

 that it acts by saving power instead of by wasting it, but in 

 addition its application imposes no tax on the driver's 

 strength, as is the case with mechanical brakes where the 

 pressure of the brake blocks is actually exerted by the 

 •driver's hand or foot — a consideration of considerable im- 

 portance in these days of the natural revolt of the " tram 

 slave." 



At each end of the Northfleet cars there are two 

 handles, one of which regulates the resistance shunting 

 the field-magnet of the motor, and which therefore replaces 

 the handle working the throttle valve of a steam loco- 

 motive, while the other handle reverses the terminals 

 •of the field-magnet, or short-circuits them when it is in its 



middle position, and therefore replaces the handle which 

 operates the link motion in the locomotive. During the 

 several journeys we made in the cars, we had freciuent 

 opportunity of seeing how perfectly they were under 

 control, even when descending the steep hill near the 

 Northfleet railway station. 



One defect of the parallel system of working electric 

 tram-lines is that it is possible for a mischievous person 

 to cut off the power from the whole line, while at 

 the same time the constant potential difference dynamo 

 is made to produce far more than its normal current, by 

 his laying an iron crowbar so as to electrically connect 

 the go'nj and return conductor. This result he may be 

 able to accomplish without much difficulty, since both 

 these conductors must be sufficiently exposed along their 

 whole length that the passing train can maintain electrical 

 contact with both of them. With the series system, on 

 the other hand, such a catastrophe is impossible, since 

 the return conductor, 3, marked "line" in Fig. 3, is 

 completely buried out of sight. 



A considerable amount of ingenuity has been displayed 

 in the mechanical details both on the cars themselves 

 and on the track at Northfleet. The motor, for example, 

 is geared directly to a spur wheel on the car wheel shaft 

 by double helical gearing, which runs without biting, and 

 at the same time without any chance of slip, since the 

 axle of the motor and the axle of the car wheel driven by 

 it are always maintained at exactly the same distance 

 apart. The result is attained by supporting one end of 

 the motor framework from two bearings on the axle of 

 the car wheel, and the other end by stout springs from the 

 car itself. The tram-line is for the greater portion a single 

 track, hence several places where the up and down cars 

 can pass, "turn-outs" as they are technically called, have 

 been provided. As both the up and down lines at the 

 turn-outs are electrically in series, special electrical 

 devices which appear to work very well have been pro- 

 vided for the facing points at the two ends of each turn- 

 out. At one portion of the line the road is so narrow 

 that it would have been very inconvenient to the ordinary 

 horse traffic, had an up and a down tramcar gone along 

 the same side of the road. An up car has therefore to 

 pass the horse traffic by following the "near side," so 

 also has a down car — in other words, the electric tramcar 

 if it be going in one direction has to be on the opposite 

 side of the road from that followed by an electric tramcar 

 when going in the contrary direction. But as the road is 

 too narrow for two sets of rails the track here consists of 

 three rails, A B and C, A and B being used by the car when 

 going in one direction, and C and B when it is returning ; 

 the facing points at the ends of this section being fitted 

 with special mechanical and electrical devices, which also 

 appear to accomplish their aims with perfect satisfaction. 



A portion of the line has a long steep gradient of i in 

 32, but instead of jaded horses having to tug the cars up 

 this hill, they ascend it at a rate of about nine miles an 

 hour, so that the whole journey, which is a little under 

 one mile, is accomplished when desired in three minutes 

 and a half. 



It is but a fcvv weeks ago that the first trial trip of the 

 Northfleet line was made ; it is but now that it is opened 

 for public use ; and yet already the passers-by and the 

 shopkeepers along the route have ceased to wonder how 

 it is that horseless cars full of people rush up hill without 

 smoke, quickly start in either direction, and as quickly 

 stop when directed by the passengers to do so. After 

 the Englishman has been spending some time conclusively 

 proving to himself that a series tram-line was a practical 

 impossibility, while the American was engaged in carry- 

 ing out the trite saying that " the best way to do a thing 

 is just to go and do it," our countrymen now accept the 

 regular daily running of the series tram-line at North- 

 fleet as a matter of course, and have forgotten that its 

 very marked success ought to astonish them. 



