568 



NA TURE 



[April 12, 1894 



and other advantages would result from turning electricity from 

 a servant into a master, from a mere transformer of energy 

 into a source of energy. 



But we have to do with things as they are : — One hundred 

 per cent, of water-power turned into a net motor power of 

 thirty-three and a third per cent, on the average of the best 

 results ; and certainly a wonderful result in itself — nothing of 

 the kind could possibly be obtained either with a steam or a gas 

 engine, and it points to the direction in which, at all events in 

 the meantime, we have to look fora cheap supply of energy for 

 electric traction. 



Electric traction may be looked upon as the most economically 

 difficult application of the electrical transmission of energy. 

 The problem of the supply of electrical energy to a factory, for 

 driving such industrial machines as lathes and sewing-machines, 

 and for its illumination when need be, from an electrical main ; 

 ihe problem of utilising a single large steam-engine within a 

 works for supplying motive power to all the different shops, so 

 as to get rid of the waste and inconvenience of shafting, gearing 

 and belting, is not without its difficulties ; but these problems 

 are, from the na'-uit' c f the case, economically simple. The work 

 that has to be done is of a uniform steady character within a 

 compact space. 



The problem of electric traction is quite otherwise ; the load 

 is now on, now off ; the electrical resistance of the line is always 

 varying w ith the distance traversed ; here there are gradients, 

 thf-re curves ; whilst the demand for cars, and the most econo- 

 mical supply of car-loads, are matters all of which have to be 

 taken into consideration, and have their influence on the 

 economy of the undertaking. 



Six systems have been employed for supplying the motor on 

 the car with electricity ; these are using the rails on which the 

 cars travel as conductors. This was done in the first under- 

 takings at Lichterfelde, and at Brighton in the first instance ; 

 having a special rail as conductor either beside or between the 

 lails on which the car travels, as used at Portrush, now at 

 Brighton, and on the City and South London underground elec- 

 tric railway ; using underground conductors with an open con- 

 duit or slot in the road, as at Budapest, Blackpool, and else- 

 where ; having an insulated underground conductor, such as 

 that laid down for a short distance near Hammersmith; over- 

 head conductors, the system which has the largest applica- 

 tion at present, and first used between Frankf>rc and 

 Offenbach, and mainly in use in the United S'ates of America, 

 where some 3000 miles of electric tramways are said to exist. 

 In the sixth system the conductors are replaced by storage 

 batteries. 



The rails on which the cars run are now seldom used for the 

 transmission of the current, as being an uneconomical applica- 

 tion, but the use of the third rail is a simple and convenient 

 method, where, as in this country, a pressure or tension above 

 500 volts is not permitter", and where a third rail on the surface, 

 as in rural districts, or in special tunnels, is not an incon- 

 venience ; in urban districts, on the other hand, underground con- 

 ductors are more suitable. There are two ways of arranging 

 the circuit in underground conductors ; either connecting the 

 positive and negative poles of the generator, to two insulated 

 underground conductors respectively, or connecting one pole to 

 the insulated underground conductors, and the other to the 

 rails, which in the latter case are in continuous conductive con- 

 nection, besides having cross connections at different points. 



The use of thin aerial conducting wires is interesting in con- 

 nection with the practical appreciation of the fact, which took 

 place some years ago, that electric energy consists of two 

 factors, viz. the electromotive force or tension and the current 

 strength. As it is to the latter only that a me'allic conductor 

 offers resistance, a certain amount of energy may be transmitted 

 through a very thin wire, if care is taken that the electromotive 

 force of the current is high enough. This is the system which 

 it is jiroposed to use on the S:. Louis and Chicago Electric 

 Railway. The di3;ance between these two towns is 250 miles, 

 and it is proposed to run the cars at the rate of 100 miles an 

 hour ; the line is to be divided into tweniy-five sections of ten 

 miles each, and in the centre of each section there will be a 

 transformer station. The current is to be generated at a tension 

 of 500 volts, transformed to a tension of 25,000 volts, at which 

 it will be transmitted to the several transformer stations, and 

 will be there reduced to 3000 volts, transmitted to the wire, 

 and thence to alternating current motors attached to the cars, 

 each driving axle being supplied with a separate motor. 



NO. 1276, VOL. 49J 



Secondary or storage batteries, which is the sixth system, 

 to which we have referred, are specially suitable for use on 

 existing tramways, on account of their simplicity and immediate 

 applicability. The objections to them are their weight and the 

 necessity of renewing them from time to time. As regards the 

 former, they weigh from li to 2 tons, and increase the weight 

 to be propelled from 20 to 25 per cent., and there seems to be a 

 real difficulty in making these batteries at the same time light 

 and durable. The batteries which are oxidised by the direct 

 action of the current are found to be too heavy for electric 

 traction, and those in which the oxide is mechanically applied 

 have been more generally used, the positive plate having a paste 

 of red lead, PbgO^, and the negative a paste of litharge, PbO. 

 The life of a storage battery depends upon its being discharged 

 at a low rate, whilst in traction high rates of discharge are 

 required. It is the positive plate which becomes disintegrated, 

 and cannot again be brought into the condition of metallic lead ; 

 the negative plate lasts for a very long time, and can then be 

 re-cast. 



This system has been employed on the Birmingham Electric 

 Tramway. On this line a method of controlling the circuit is 

 in use, obviating the necessity of inserting lesistances in the 

 circuit, which is always a wasteful way of employing the current. 

 By this means the driver is able with a switch handle to 

 apply from a fourth to the whole of the power which the 

 batteiies can produce ; the whole would be available when 

 required to start the tram, or when going uphill with a heavy 

 load, whilst a fourth would be used when travelling on the level 

 with a light load. There are two other positions in which a 

 half or three-quarters can be used as required. 



A consideration of importance in connection with electric 

 traction is the style of motor to be used ; it should be able to do 

 its work equally well when starting, when after having started 

 it is under the influence of the full current, or where at the 

 further end of the line the current is reduced by the resistance 

 of the conductor; it must be able to be stopped in a moment, 

 and work efficiently with all variations of speed and load. For 

 these purposes shunt motors are not so suitable as series motors, 

 in \Nhich the field-magnet coils are connected up in series with 

 the armature, and hence the latter and compound wound motors 

 are those most frequently used. Its special design depends upon 

 the space at disposal in which to fix it, and upon the gearing — 

 whether spur, chain, or worm gearing — used to transmit its 

 motion to the driving axle of the car. In some instances it is 

 advantageous to drive each pair of wheels by a separate motor. 



In conclusion, it may be stated that the present conditions 

 under which electric traction is carried on are altogether in its 

 favour, namely, light tramway cars following one another in 

 frequent succession, and travelling at a moderate speed. 

 What is proposed to be done on the St. Louis and Chicago 

 Railway is something quite different. It will probably be 

 difficult to find passengers enough to fill cars to follow one 

 another at a sufficiently quick rate to make the undertaking 

 pay. And then as regards the very high speeds proposed to 

 be used there and elsewhere, one hundred up to two hundred 

 miles an hour, the cars will have to t)e made of special forms 

 to resist the enormous pressure of the air at these high velocities, 

 and specially ventilated ; whilst the effective horse-power 

 required to be supplied by the motor will greatly increase the 

 weight of the car. E. F. Bamber. 



ED UCA TIONA L A GRICUL TURA L 

 EXPERIMENTS. 



'T'HE Agricultural Research Association of Scotland was 

 ■*■ founded for the purpose of obtainmg trustworthy and use- 

 ful information on agricultural subjects, by means of scientific 

 investigation and practical experiments. The report of the 

 committee for 1893 shows that the work of the association con- 

 tinues to proceed satisfactorily. Research work is always 

 difficult to maintain, and a committee fostering it must regard 

 It as an unavoidable, though unpleasant, duty to press its claims. 

 •' It is a matter of much regret to the committee," we read, 

 " that we should have so constantly to press for adequate means 

 -o carry out the work, and that progress should so constantly be 

 checked for want of means. But the reason is obvious, for an 

 Association confining itself to investigation holds a peculiar 

 jiosition. It is different, on the one hand, from organisations 

 ' hat are enabled to return interest for the money spent ; and, on 



