throwing a particular machine out of circuit, but a like disconnection also occurs should any accident happen to either the engine or the machine.

Compound machines are also sometimes worked in parallel, but the arrangement is necessarily more complicated, and the liability to accident when making changes greater, than in the case of shunt machines. The method of connecting devised by Mr. W. M. Mordey and illustrated in fig. 173 is probably the best, and is certainly satisfactory when the machines are similar. Two machines are shown in the figure, connected across between the positive and negative main leads, from which the lamp circuits may be tapped off, the object being, of course, to enable the load to be equally divided between the machines. One end of each of the series coils F, F, is shown connected to the positive main, the other ends being directly joined together by the wire P Q end of the shunt coil s, is also connected to P, and the corresponding end of the shunt coil Są to Q, the other ends being joined to the negative main. The points P and Q are always at the same potential, or, if not, an equalising current will at once flow between them, and the consequence is that the difference of potential between the ends of the two series coils is always practically the same, and the currents passing through them must be equal if their resistances are equal; and also the potential difference between the ends of the two shunt coils is the same and the currents in them will be equal if their resistances are equal. Therefore the strength of field will be the same for both machines, and they will equally share the work when driven at the same speed.

Should the machines be unequal the matter is not quite so simple, as among other things the resistances of the shunt and series coils of the two machines must be so proportioned that each will get the requisite amount of current to excite the fieldmagnets up to their proper value.

It is evident that as the object of the wire P Q is to keep the two points P and Q at the same potential, it must be of sufficiently low resistance to prevent any appreciable fall of potential along it, notwithstanding the flow of an equalising current of considerable strength.

B1, B2, C1, C2 and D represent switches to facilitate the throwing in and out of circuit of either machine. Suppose A, is to be brought into play while A, is working, c, and then D should be closed so that currents flow in the proper direction through S, and F2 to excite the field-magnet coils, and then, the armature being run up to the required speed, its switch B, may be closed, and the machine then takes up its proportion of the work.

CHAPTER X.

DIRECT CURRENT DYNAMOS-continued

WE will now illustrate and describe some of the best modern direct-current dynamos, directing attention in each case to those details which are likely to prove most instructive in showing how theoretical principles are applied, and how, in practice, mechanical and economical considerations sometimes cause a deviation from forms which a narrow theory might show to be the best.

The student is recommended to pay particular attention to the methods adopted for securing mechanical strength and durability. It may be observed, for example, that it is quite as important to prevent the conductor being stripped from an armature as to efficiently insulate it. Again, while a waste of power, such as is evidenced by the heating of the iron core by eddy currents, is to be deprecated, any waste which shows itself in such a manner as the undue heating of bearings is equally, or even more, to be avoided. In both cases the power applied to the shaft is wasted in the former case after, and in the latter case before, it has been transformed into electrical power.

Fig. 174 illustrates a dynamo constructed by Messrs. Easton, Anderson & Goolden, the armature being of the Gramme ring type, and the field produced by an inverted horse-shoe magnet. This particular machine is compound-wound, and the arrangement is therefore similar to that depicted in fig. 169. A vertical section of the machine at right angles to the shaft is given in fig. 175. The bed-plate is of cast-iron and includes a solid piece, D, in the centre, directly under the armature and field-magnet, to form the

yoke of the latter. But wrought-iron is employed for that portion of the cores round which the field-magnet coils are wound, each core, w w, consisting of a slab of soft hammered scrap-iron; thus giving the advantage, previously referred to, of economising copper wire, by obtaining the requisite magnetic conductivity with the minimum sectional area. The pole-pieces, P P, are of grey cast-iron, and the sectional area of all the cast-iron portions is

made greater than that of the wrought-iron portions to compensate for the lower permeability as compared with that of the excellent iron forming the core. The cast-iron pole-pieces are here tapered away to the top, but it is found that any considerable reduction of the area of the magnetic circuit in this manner tends to make the field through the armature stronger at the lower part than at the upper, with the result that there may be, especially in large