VOLTAK BLEl TIUC1TY. 



;:, ; 



Our other illustration, Kiif. H'J, represent* a modern Siemen*' 

 miu-hine, which is largely employed in thia and other con 

 The detail* which we have given with regard to the Gramme 

 ring will apply to the armature used in this machine, which, 

 although not identical with the Siemens' armature, adverted to 

 in a former lesson, exhibit the same principle in a modified 

 form. In the old form of armature, the currents given were 

 subject to alight variations ; but in the multiple armature now 

 under consideration, this difficulty is obviated. The core upon 

 which the coils are wound is a hollow iron cylinder, and the 

 magnetic poles induced in it maintain a constant relation to the 

 pole* of the magnets, a* in the Gramme ring. There are usually 

 in this machine four field magnets two pair* with their north 

 poles near together, and two on the opposite side of the arma- 

 ture, with their south poles adjacent so that really there is 

 one north and one south pole to act upon the revolving arma- 

 ture. 



Although there is thus some resemblance between the 

 Gramme and Siemens' armatures, there is one point of impor- 

 tant difference. The Gramme ring is wound in and out with 

 the coils of wire, whilst the Siemens' armature is wound wholly 

 on the outside. We 

 have chosen these two 

 machines for illustra- 

 tion, because they repre- 

 sent types which most 

 other machines follow. 

 Thus the largely em- 

 ployed Brush machine 

 has a ring armature, 

 which differs only in 

 points of detail, which 

 are certainly consider- 

 able, from the Gramme. 

 In Jablochkoff's machine 

 we find a modification of 

 the alternate current 

 Gramme described 

 above. In the Burgin 

 machine, again, we find 

 another armature built 

 on the direct current 

 model, but with impor- 

 tant differences ; and so 

 on with different ma- 

 chines without end. In 

 like manner the Siemens' 

 machine has found ad- 

 mirers who have been 

 pleased to regard it as a 



model which they could alter, and perhaps improve. In the 

 enormous dynamo constructed by Edison, and destined to 

 furnish energy for the illumination of extensive districts, we 

 find a distinct modification of the Siemens' multiple armature, 

 only the wires give place to flat bars of copper about half 

 an inch broad, which are connected together by discs. The 

 large magnets employed in the Edison dynamo machine weigh 

 several tons, and the general appearance of the monstrous 

 contrivance will be familiar to those who visited the late elec- 

 trical exhibition at the Crystal Palace. The wonderful and 

 splendid display of lights which owed their power to the 

 machine in question, at once proved that it was efficient for the 

 purpose for which it was designed. 



In studying the phenomena by which dynamo machines are 

 governed, we must not so much regard the condition of the 

 wires and magnets, as the space which surrounds them, for to 

 that space we must look for the energy represented by the 

 magnetic attractions. We can in a manner view the condition 

 of this space, or magnetic field as it is called, and by very 

 simple means. We need only a magnet and any little toy 

 magnet will do for the purpose a sheet of cardboard, and a 

 few iron filings contained in a piece of muslin. Laying the 

 magnet flat on the table, we place the card above it, and scatter 

 over the white surface some filings from our little muslin bag. 

 When the card is pretty equally covered with the dust, we can 

 gently tap it at one corner with the finger-nail, when we shall 

 see a strange and beautiful sight. The little particles of iron 

 marshal themselves into apparently regular figures, forming 



curves of beautiful feathery outline, which Mem to spring from 

 the two magnet poles below, and to bend toward* each other in 

 symmetrical lines. These cnrree are known M the curve* of 

 magnetic force, which exist in space. round the pole* of any 

 magnet. Even a steel knitting needle which ha* been map- 

 netUed by a few touches of a permanent magnet will exhibit 

 these curves. We may vary the experiment by placing the mag 

 net in different positions, or by placing piece* of metal in it* 

 vicinity, and by such experiment* we can learn much of tbe 

 mysterious force called magnetism. By dusting the filing* over 

 a piece of glass which has been previously covered with gum 

 water and thoroughly dried, instead of using cardboard, and by 

 afterwards breathing upon the glass so a* to soften the gum 

 and fix tbe iron filings in the positions they have taken np, we 

 can obtain a most interesting slide for projection by mean* 

 of a lantern. In all dynamo machines the armature* are placed 

 as close as possible to the field magnets, so that a* they tarn 

 they cut the lines of force. Thus in our two illustration* 

 of the Gramme and Siemens' machines, we find in the first that 

 the magnet ban projections or cheek-pieces which closely em- 

 brace the ring nrmatnro, and that in the other case the magnets 



are so bent as to closely 

 surround the moving 

 coils. 



We have seen in a 

 former lesson that the 

 voltaic battery cell is in 

 a manner reversible 

 that is to say, it can be 

 so constructed as to fur- 

 nish a secondary current 

 after being charged by 

 an ordinary cell. Such 

 arrangements are called 

 secondary batteries or 

 accumulators, and a 

 great deal is expected 

 from their future deve- 

 lopment. A still more 

 important discovery is 

 represented by the cir- 

 cumstance that the 

 Gramme and kindled 

 machines are reversible. 

 In other words, any dy- 

 namo machine giving a 

 direct current can be 

 made to act as an electro- 



' motor. Thus, supposing 



we take the little machine 



shown at Fig. 81, and connect it with an ordinary Bunsen orGrove 

 battery. It will immediately begin to turn round on its own ac- 

 count, but in the reverse direction to that it would have if we were 

 using it for ordinary experiments. It is clear, therefore, that if 

 by means of a belt we were to connect the machine with a lath", a 

 sewing machine, or any contrivance where power is required, we 

 could easily set such contrivance in motion. But, as we hive 

 already shown that the Gramme machine acts in all respects like 

 a battery in its current-affording properties, it is obvious that by 

 coupling np two such machines together, and by turning one of 

 them, the other will immediately be set in motion. Thia dis- 

 covery of the reversibility of the dynamo machine has been re- 

 garded by many eminent electricians as one of the most important 

 discoveries of the nineteenth century : but this hi .-h estimate 

 of its importance will not at once be rendered evident to the 

 casual reader, but a few considerations will serve to justify it. 



We are constantly hearing of new applications of electricity, 

 and recent years have brought forward electrical railways, tram- 

 cars, launches, and even tricycles. Now, one and all of these 

 depend for their existence upon the discovery we are consider- 

 ing. In other words, they employ a dynamo machine as & 

 motor, instead of steam or muscular power, as the case may 

 be. Taking the case of the electrical launch, which was 

 tried for the first time on the Thames, we find an ordinary 

 boat such as is usually propelled by steam. But instead 

 of the best part of the space being occupied by a boiler and 

 engine, all is clear. There is no visible machinery, for it is all 

 hidden beneath the boarding. Could we look inside, we should 



