regulator, which is very ingenious and effective, is due to Wood. This consists of an electromagnet, which controls a mechanism that shifts the brushes on the commutator as lamps are cut out of circuit, until finally, when the machine is short-circuited, the brushes are almost 90° from their normal position, and the E.M.F. becomes very small. The violent sparking which ordinarily occurs when the brushes are displaced from the neutral points is avoided in this dynamo by practically balancing the M.M.F. of the field by that of the armature, so that wherever the brushes may be, practically no E.M.F. is generated in the coils which are shortcircuited by them. This is shown very clearly in the diagrams

given by Professor R. B. Owens in a paper on a "Test of a Closedcoil Arc Dynamo," which contains the results of very careful tests on the action and regulation of a 25-light Wood arc dynamo. These machines are made in various sizes up to, and including, 125-light capacity.

The Excelsior (Hochhausen) Arc Dynamo is a well-known type, in which the armature is a closed-coil ring, with quite a large number of sections in the commutator, as in the case of the Wood machine. The field-magnet is of the peculiar form shown in Fig. 121; the fronts of the pole-pieces being carried on hinges, which permit them to swing outward, as represented, to give access to the armature. The current regulator comprises an

* Trans. Amer. Inst. Elec. Eng., May, 1894.

electromagnetic controller placed near the machine, and a small electric motor mounted on the latter. This motor has a pinion on its shaft, which engages with a semicircular rack attached to the rocker-arm, that carries the brushes of the dynamo. The controller acts to send current in one direction or the other through the armature of the motor if the main current rises above, or falls below, its normal value, thereby shifting the brushes until the current is brought back to the proper strength. The rockerarm is connected by a rod to a switch that cuts into or out of circuit sections of the field-winding, also assisting in the regulation.

This type is manufactured in various sizes, but is especially designed for high voltage, a 200-light machine, generating an E.M.F. of 10,000 volts, being described in the Electrical Engineer (N.Y.), May 30, 1894.

ALTERNATING-CURRENT DYNAMOS.

The armature windings employed in alternators have already been described in Chapter XVII.; and in general they are simpler than those required for direct currents, since they consist merely of a series of coils corresponding in number to the pole-pieces. The field-magnets are usually quite similar to each other in form, as they are almost necessarily multipolar in order to obtain the necessary frequency, which varies from 25 to 140 periods per second; but in this country is ordinarily between 125 and 140 for electric lighting.

Constant-Potential, Single-Phase Alternators are almost universally made according to the general design shown in Fig. 122 in this country, and also to a great extent abroad. The fieldmagnet resembles the form widely adopted for multipolar, directcurrent machines (Fig. 116); almost the only difference being the fact that in the latter the space between adjacent pole-pieces is only one-third to one-half of the arc covered by each pole-piece, whereas in alternators the spaces and pole-pieces are usually equal in extent. Alternators similar to the form shown are manufactured by the General Electric, Westinghouse, and Fort Wayne Electric Companies, which include about nine-tenths of the single-phase alternators in use in the United States. The fieldmagnetizing current is usually obtained from a small auxiliary direct-current dynamo called an "exciter."

These machines are also often made with composite field-winding, which is analogous to compound winding for direct-current dynamos, and consists in providing the field-magnets with a few turns of coarse wire in addition to the fine wire winding fed by the exciter. The main current generated by the machine is passed through this coarse winding, being rectified or converted into a direct current for the purpose by means of the commutator shown on one side of the collecting-rings in Fig. 122. In this way the voltage may be kept constant, or raised with increased load. Instead of passing the main current itself through the extra coils, it is possible to obtain the same effect by means of a transformer in which the main current acts inductively upon a secondary circuit, and produces a current proportional to itself, which is rectified and used in a similar way.

The Mordey Alternator is a well-known type manufactured in England, and embodies several interesting features. The armature, which is stationary, consists of a number of flat coils that do not contain any iron core. The field is mounted upon a shaft, and revolves. It has only a single coil, to which the magnetizing current is conveyed by contact-rings on the shaft. There are a number of projections from one end of the field-magnet which are all of north polarity, and an equal number of south polarity on the other end. The armature stands in the space between these poles; so that when they revolve each armature coil is subjected to a rising and falling magnetic flux, which does not, however, reverse, since the north poles are always on one side of the armature, and the south poles on the other. Nevertheless, this generates an alternating current, since increasing and decreasing the flux through a coil produces E.M.F. in opposite directions. A similar form of machine is built in the United States by the Brush Electric Company.

Other prominent European types of alternator are built by Ferranti, Ganz and Company, and other manufacturers.

Polyphase Alternators belong rather to electric power than to electric lighting, as stated on page 288, where their general principles were explained. Hence typical examples of them will not be described herein.

CHAPTER XIX.

THE PRACTICAL MANAGEMENT OF DYNAMOS.

THE actual handling of the dynamos in a central station or isolated plant is an especially important matter in electric lighting. A work by Dr. S. S. Wheeler and the author 1 is devoted to the selecting, installing, operating, and testing of dynamos and motors, special attention being given to the serious problem of locating and remedying troubles in these machines. The reader is referred to that book for a complete treatment of these subjects, as limitations of space will only allow the principal points to be given in the present chapter.

The Selection of a Dynamo. One of the first questions that the electrical engineer is called upon to decide is the selection of a dynamo for a certain plant. It depends largely upon circumstances in each particular instance, but there are certain general principles which apply to almost all cases.

Construction. This should be of the most solid character, and first-class in every respect, including materials and workmanship.

Finish. A good finish is desirable, first, because it indicates good construction; second, it stimulates the interest and pride of the attendant; and third, it shows the least dirt or neglect. Simplicity. The machine and all its parts should be as simple as possible, and any peculiar or complicated feature should be avoided. These are sometimes successful, but should be well tried and proved before being accepted.

Attention. The amount of attention required by the dynamo should be small. The screws, connections, and other small parts should be arranged so that they are not likely to become loose, and the delicate parts should not be exposed or liable to injury.

1 The Practical Management of Dynamos and Motors, by F. B. Crocker and S. S. Wheeler, Third Edition, D. Van Nostrand Co., N. Y., 1894.