and the gross E.M.F. deduced therefrom. As the range in a single coil is usually from 100 to 150 volts, an ordinary voltmeter can be used for the purpose, whereas, to measure the total potential difference directly, an electrostatic or other special voltmeter would be required. The machine is therefore fitted with a special pair of voltmeter terminals, connected to the ends of one coil, for the purpose of making this measurement.

1 It will be observed that the arrangement of the armature coils can, if required, be readily altered to vary the E.M.F. and current developed. For example, if it were required to reduce the E.M. F. of a 2,000-volt machine to 1,000 volts, this could be done by dividing the armature coils into two sets and joining them together in parallel, the machine being then capable of developing twice the current strength with the same density of current in the armature coils. Normally the larger machines are so connected with the armature coils in two sets joined in parallel, but the usual arrangement for the smaller sizes is with all the coils in series, in which case, of course, special attention must be paid to the insulation of the two adjacent coils at the terminals, since the maximum potential difference exists between them. In any case the total electrical output, or the energy developed, would be the

same.

Fig. 144 represents the field-magnet of an early type of machine, the figure being retained because it illustrates the general principle more clearly than would a view of the latest pattern. In these earlier machines, copper dishes were attached outside the cast-iron claws of the field-magnets, for the purpose of reducing the amount of air-churning which would otherwise occasion loss of energy. The claws, in fig. 144, are shown without these dishes; in the newer form of the machine they are dispensed with, the claws being simply webbed together in the casting, when they present the appearance shown in fig. 142.

The field-magnet is excited by the current from a small Victoria direct current dynamo, which is mounted on a bracket projecting from the main bed-plate, its shaft being coupled direct to the alternator shaft, so that the two machines are driven together. A long thrust-bearing is employed to prevent end-play, which is an important point, since the space between the pole-faces and

armature is very small; and this bearing is adjustable longitudinally for the purpose of enabling the field-magnet to be symmetrically disposed with regard to the armature. The right-hand end of

the shaft shown in fig. 144 fits into this bearing. The armature terminals are placed on the upper portion of the gun-metal supporting ring.

The machine illustrated, when driven at 500 revolutions per minute, is capable of developing 75,000 watts, or 100 electrical horse-power, at an E.M.F. of 2,000 volts. 900 watts are required for the purpose of exciting the field-magnets. On account of there being no iron in the armature, and the attention devoted to small details such as the use of German silver for the coil-fittings, the waste of power due to hysteresis and eddy currents is very small ; and this loss, added to that due to friction, which, owing to good mechanical construction, is also very low, amounts to but 5 horsepower, that being the power required to drive the machine at full speed on open circuit (or when the armature is disconnected), the field-magnets being at the same time excited to their maxi

mum.

In many cases the output demanded from a dynamo varies considerably at different times. For instance, four times as much power may be required to supply lamps at one time as at another. It is not economical to use one large machine, capable of meeting the maximum demand, and run it to give a small output at other times; but, fortunately, it is possible to join up two (or even more) alternating-current dynamos so as to feed the same circuit simultaneously when required, switching out and stopping one when the other is able to meet the low demand.

The armatures must not be joined up in series, but in parallel, and the machines may be driven by belts from the same shafting, or, if necessary, from independent engines running at about equal speeds. In practice the latter course is usually adopted, since it is uneconomical to employ a large engine to develop the power required by a small machine, which would be the case if two or more machines were driven from a common countershaft driven in its turn by a single large engine.

But parallel working is only practicable when in both machines the rates of alternation are equal, and the alternations 'co-phasal'

-that is, when their maximum and likewise their minimum E.M.F.'s occur simultaneously. It is most remarkable that welldesigned machines can correct each other and maintain this synchronism; but, as a most important part of the interaction depends upon the 'motor' properties of a dynamo, further consideration of the question must be deferred until electric motors have been dealt with.

297

CHAPTER IX

DYNAMO-ELECTRIC MACHINES (DIRECT CURRENT)

ALTHOUGH the sphere of usefulness for alternating-current dynamos has largely increased of late years, there is still a vast amount of work which such machines are, and always will be, wholly incompetent to perform. This is notably the case in connection with the deposition of metals by electricity, and in the 'charging' of secondary batteries. For these, and several other important purposes, it is essential that the current should be continuous, and flow in one direction only. Except in the case of a few experimental machines, the current which is generated in the armature always alternates in direction; but it is possible to arrange matters so that all the currents so generated shall be made to flow in one direction in the external circuit, the process being known as commutation,' and the part of the machine by which the alteration is effected is termed the 'commutator.' Directly this has been successfully performed, the dynamo is capable of a new and important development, for it is then possible to use all or a part of the current which is generated in the armature, for the purpose of magnetising the field-magnets. The smaller auxiliary machine, which, in most of the dynamos previously described, has been employed to excite the field-magnets, can therefore be dispensed with, and the machine made 'self-exciting.'

We come then to the consideration of the means to be employed in order that the currents which are generated in alternate directions in the armature itself can be commutated so as to flow in one direction in the external circuit. Referring again to fig. 127, we remember that the direction of the current is unaltered

(although it varies in E.M.F., and therefore also in strength) during the first half-revolution of the rectangle, and that, at the end of that half-revolution, the reversal in direction takes place. Now, a moment's reflection will show that if, just at the end of this first half-revolution, the positions of the two brushes on their respective rings were instantaneously interchanged, the current generated during the second half of the revolution would flow in the same direction round the external circuit as the preceding current did, because, although really generated in the reverse direction, it is entering the external circuit at the other end. This is the fundamental principle of commutation; only, instead of shifting the brushes, the change is effected at the right moment by a modification of the ring or rings against which the brushes press.

α

FIG. 145.

b

The simplest possible form of commutator is shown in section in fig. 145. Instead of two brass rings, a single brass ring or tube is employed, but with the difference that it is split lengthways into two halves or segments, a b, insulated one from the other. Each end of.the rotating coil of wire is connected to one of these segments, and the brushes or flat springs (which are permanently connected one to each end of the external circuit) are so situated that they press upon the insulating divisions between the segments at the moment that the coil is in the vertical position - that is to say, in the position where the reversal of the current takes place. Just at that moment, then, the ends of the coil in contact with the respective brushes are also reversed, and the result is that when the coil is rotated uniformly, a succession of short currents passes through the external circuit, each current rising and falling similarly, but all impelled through the external circuit in the same direction.

The length of the wire can easily be increased by winding it in a number of convolutions, instead of in a single rectangle, when, as a matter of course, the E.M.F. will be increased proportionately.

The variation in the E.M.F. developed by an ideal alternatingcurrent dynamo is shown in fig. 126, where the line A B represents