152 



THE POPULAR EDUCATOR. 



VOLTAIC ELECTRICITY. XIII. 



MODERN DYNAMO-ELECTRIC MACHINES THOSE OF GRAMME, 



SIEMENS, ETC. TRANSMISSION OF POWER BY ELECTRICITY 



ELECTRICAL RAILWAYS. 



UP to the time of Siemens' and Wheatstone's proposition that 

 there existed naturally in iron sufficient residual magnetism to 

 initiate a current in a revolving armature placed in proper re- 

 lation to an electro-magnet, permanent steel magnets had been 

 almost universally employed in the construction of magneto- 

 electric machines. "With the exception of one or two machines 

 which still employ them, permanent magnets have gone out of 

 use, and are replaced by the far more powerful electro-magnets. 

 Modern machines employing these latter are now known as 

 dynamo-electric machines, to distinguish them from those which 

 employ permanent magnets. But it must be understood that 

 both classes of machines are urged into action by the dyna- 

 mical power of a steam-engine, a gas engine, or other kind 

 of motor. 



In Fig, 81 we have a representation of the celebrated Gramme 

 machine, and although this particular form of it is intended 

 merely for demonstration, and is worked by hand, the rela- 

 tion of its various parts can, perhaps, be better under- 

 stood than if we placed before the 

 reader one of the larger machines 

 designed for steam power, which are 

 now in such extended use for various 

 purposes. The power of this little 

 lecture-table machine is equal to 

 eight ordinary Bunsen cells, and as 

 it can be put in motion at any time 

 without the tiresome preliminary of 

 charging a battery, it is a great boon 

 to experimenters. The magnet, in- 

 stead of being one solid piece of steel, 

 is made up of steel ribbons, as advo- 

 cated by Jamin, by which greater 

 power is obtained. But the essential 

 and most important feature of the 

 Gramme machine is the armature. 

 This, instead of being of the bobbin 

 or shuttle form, is ring-shaped. To 

 understand its action, let us suppose 

 that we have before us a straight 

 bar magnet, and that we slide upon 

 it a coil of wire from left to right. 

 Following the passage of this coil as 

 we slowly drag it along the mag- 

 net, we can easily understand that 



a current will be induced in it as we move it in the vicinity ! 

 of the foremost pole. As soon as the coil passes the centre of 

 the bar, which we may look upon as a neutral line, it will 

 become subject to another induced current, but in the reversed 

 direction. Continuing the experiment with two bar magnets 

 placed end to end, in contact by poles of the same name, we 

 shall find that when the coil traverses the combination it will 

 have induced in it at first say a positive current, until it is near 

 the point of junction, when the current will be negative to be 

 positive once more when the coil approaches the end of its 

 course. Now, if we imagine two such bars bent into half 

 circles, so that when their ends are in contact they form a 

 perfect circle, precisely the same induced currents will traverse 

 a coil moving upon them. 



To still better elucidate the action of the Gramme armature, 

 let us further suppose that we take a ring formed of soft iron, 

 and wind upon it an insulated copper helix or coil. This helix 

 is quite continuous, its extremities being soldered together so 

 as to form an endless wire. Now let us see how such an 

 arrangement behaves when revolved between the poles of a 

 magnet. The soft iron is of course magnetised by induction, 

 and the side of the ring nearest to the N pole of the magnet 

 will have developed in it a south pole ; the opposite side, next 

 to the S of the magnet, being north. As the ring turns, these 

 poles developed in it maintain the same relation to the magnet 

 poles, and therefore suffer displacement in the iron itself so 

 long as the movement of the armature continues. The copper 

 helix becomes subject to currents having two contrary directions, 

 one half being traversed by positive currents, and the other 



half by negative currents. Now it is obvious that by dividing- 

 such a helix into sections we shall obtain several currents, and 

 that it will be possible to gather up these currents much in the 

 same way as we can obtain the combined effect of several 

 battery cells. 



In the Gramme armature, the interior is formed of a bundle 

 of iron wires, which can be magnetised and demagnetised more 

 quickly than if a solid core were employed. And upon it is 

 wound in sections insulated copper wire, the ends of which are 

 brought to the axis upon which the ring revolves, but which 

 still remain insulated from one another. Referring once more 

 to Fig. 81, we can see how two metallic brushes rub against 

 this cylinder, one above and the other below. The duty of 

 these brushes is to collect the induced currents, one taking off 

 the positive and the other the negative. The current thus 

 afforded is like that from a battery, for its direction is constant ; 

 whereas, it will be remembered, that in machines of the old type 

 the currents were alternate in direction, and required the help 

 of a commutator. 



The introduction of the Gramme machine may be looked upon 

 as opening a new and very important chapter in the history of 

 electricity. The older machines had been brought to a certain 

 pitch of perfection, and progress seemed there to stop. They 

 were adopted for a few lighthouses, 

 but their cumbrous nature and the 

 large amount of power required to 

 set them in motion, limited their use 

 almost solely to that purpose. There 

 was then a lapse of a few years, 

 during which the magneto machine 

 seemed to be almost forgotten, when 

 suddenly the Gramme machine was 

 brought forward. Being the inven- 

 tion of a Frenchman, the machine 

 was naturally adopted in France long 

 before it was seen in England. We 

 believe its first appearance here was 

 in 1873, when it was used for the 

 purpose of producing an electric 

 light on the summit of the clock- 

 tower at Westminster. But in 1878, 

 at the time of the Great Exhibition 

 at Paris, all the world was invited 

 to see how the Gramme machine 

 could be adapted for lighting streets 

 and large public buildings. From 

 that time the science of electricity 

 has taken a fresh lease of life ; and 

 we may safely say, that between that 



date and the present time more has been added to our know- 

 ledge of the science than could have been reasonably looked 

 for in three times the period. 



It is right to mention, that the principle of the Gramme ring 

 was discovered as early as the year 1860, by an Italian named 

 Pacinotti ; and a rough model of the original machine con- 

 structed by him attracted a great deal of interest at the late 

 Paris electrical exhibition. But there is no question that 

 Gramme re-discovered the same principle quite independently, 

 and without any knowledge of his Italian forerunner. As we 

 have seen, the current given by the Gramme machine is con- 

 tinuous, and this is of great advantage for many purposes, and 

 indispensable to some, such as the electro deposition of metals. 

 But the machine is sometimes made to give alternating cur- 

 rents, which of course can easily be done by tapping the ring at 

 the right places ; for we have already seen that currents of 

 contrary direction exist in it at the same time. This modifi- 

 cation of the Gramme machine has been adopted for the Jab- 

 lochkoff lights on the Thames Embankment and elsewhere, for 

 that system of lighting requires alternate currents. Indeed, 

 when the Jablochkoff plan was first tried in London, an old 

 Alliance machine was unearthed from somewhere, because no 

 machine of the modern type could be found that would answer 

 the purpose. In this modified form, the ring is generally made 

 stationary, the magnets revolving within it. It is, of course, 

 merely a matter of convenience whether the coils or the magnets 

 shall be the movable parts of these machines, and usually the 

 coils, being the lighter bodies, are the parts which are caused 

 to rotate. 



* 



81. 



