Whenever a current is not distributed equally in the cross-section of any conductor there is a real increase in the resistance it offers; the heating effect being a minimum when equally distributed. The fact that the oscillatory currents are greatest at the skin gives the strongest support to the modern view that the energy in an electric circuit is transmitted by the surrounding medium and not through the wire (see Art. 519 on energy-paths).

477. Alternate-current Electromagnets. When an alternate current is sent through a coil it produces an alternating magnetic field. An iron core placed in the alternating field will be subjected to a periodic alternating magnetization. Electromagnets for alternate currents must have their iron cores laminated to avoid eddy currents; and owing to their choking action are made with fewer turns of wire than if designed for continuous currents of equal voltage. They repel sheets of copper owing to the eddy currents which they set up in them; the phase of these eddy currents being retarded by their self-induction. Elihu Thomson, who studied these repulsions, constructed some motors based on this principle. A solenoid, with a laminated iron plunger, if supplied with alternate currents at constant voltage, has the remarkable property of attracting the core with much greater force when the core is protruding out than when it is in the tube. This also is owing to the choking action.

LESSON XLIV. —Alternate-current Generators

478. Alternators. The simple alternator (Fig. 243), with its two slip-rings for taking off the current, is merely typical. In practice machines are wanted which will deliver their currents at pressures of from 1000 to 5000 volts, with frequencies of from 50 to 120 cycles per second.

Slower frequencies are unsuitable for lighting, though applicable for power transmission. High voltages are common with alternate currents because (when using transformers) of the economy (Art. 447) thereby effected in the copper mains. Under these conditions almost all alternators are designed as multipolar machines; and as the perfect insulation required in the armatures is more readily attained if these parts are stationary it is common to fix them, and instead to rotate the field-magnet. The latter is separately excited with a small continuous current led in through slip-rings. One advantage of alternate current machines over continuous current dynamos is that there is no commutator.

Amongst the various types of alternators may be mentioned the following:-(1) Magnet rotating internally and consisting of a number of poles, alternately N and S, pointing radially outwards; armature external, fixed, and consisting of a number of coils wound either upon an iron ring (Gramme), or upon inwardly projecting iron poles (Ganz), or set against the inner face of an iron core (Elwell-Parker), or embedded in holes just within the face of an iron core (Brown). In all cases where iron cores are used in armatures it is carefully laminated. (2) Magnet fixed externally and consisting of a number of alternate poles pointing radially inwards; armature internal, revolving, consisting of a number of coils wound either upon the surface of a cylindrical iron core (Westinghouse, Thomson-Houston) or fixed upon radially projecting poles (Hopkinson). (3) Magnet fixed externally and consisting of two crowns of alternate poles, alternately N and S, projecting toward one another and nearly meeting, so making a number of magnetic fields between them; armature revolving, and without iron, consisting of a number of flat coils mounted together as a sort of star disk, revolve in the narrow gaps between the poles (Siemens, Ferranti).

Another form, known as Mordey's alternator, largely

used in England, is depicted in Fig. 259. The thin armature coils are fixed, in an external stationary ring, between two crowns of poles revolving on each side of them. These poles are, however, all N poles on one side, and all S poles on the other, being projections of two massive iron pole-pieces fixed on the shaft against a huge

internal bobbin, thus constituting a solid simple form of field-magnet. On the end of the shaft is a small continuous-current dynamo as exciter.

In Fig. 260 is given a view of the central generating station for the electric lighting of the City of London. Two kinds of alternators (Thomson-Houston and Mordey) are used. The cut shows one of the latter driven by an 800 horse-power steam-engine. Each of these machines has 40 poles in each crown, and can deliver 250 amperes at 2200 volts.

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479. Coupling of Alternators. In the use of two or more alternators on one circuit a peculiarity arises that does not exist with continuous-current dynamos, owing to differences of phase in the currents. If two alternators driven by separate engines are running at the same speed and at equal voltage, it will not do to join their circuits by merely switching them to the mains if they are not also in phase with one another; or serious trouble may occur. In central station work it is usual to run several machines all in parallel. Now if two machines are feeding into the same mains each is tending to send current back to the other; and if their electromotive-forces are at any instant unequal, that with the greater will tend to send its current the opposite way through the other. To explain what occurs consider Fig. 261, which is a revolving diagram of the same kind as Figs. 251 and 254. If the two alternators are exactly in step, they will both be sending a pulse of current toward the mains at the same moment, but, so far as the circuit connecting them is concerned, these impulses will be exactly opposed. Let OA and OB represent these two exactly opposed impulses. Now suppose one of the two machines to gain a little on the other, OA shifting forward to OA'. The two electromotive-forces no longer balance, but will have a resultant OE tending to make a current oscillate through the two machines, this current being out of phase both with the leading machine A and with the lagging machine B. But this local current will itself lag a little in phase behind OE because of the inductance in its path. Let the phase of the current then be indicated by OC, which is set back a little. There is now a current surging to and fro between the two machines, and it is obviously more nearly in phase with

B

Fig. 261.