sists of a series of thin wrought-iron discs insulated from each other at their outer periphery, and supported on the spindle by metal arms in a positive mechanical manner. The wire coils are, however, held on the core by friction only, which is increased by the presence of the usual binding hoops. The electrical data of a machine intended for a current of 200 amperes at 110 volts pressure, as given by "The Engineer" for Aug. 7, 1885, are as

follows:-Magnet cores, 74 inches diameter; length of magnetizing coils (which are wound on separate metal sleeves), 12 inches; armature core, 12 inches diameter and 12 inches long; armature conductor, 203 mils solid wire wound in 120 convolutions, and connected in the usual way with a 40-part commutator; resistance of armature, 023 ohms; field magnets are compoundwound; resistance of shunt coils on magnets, 19.36 ohms; resistance of main coils on magnets, 012 ohms; each magnet limb contains 1,680 turns of 65 mils shunt wire, and 42 turns of treble 203 mils main wire; normal speed, 1,050 revolutions a minute. There is no provision made for ventilating the interior of the armature core.

The core of the Elwell-Parker Armature consists of iron wire coiled direct upon two sets of metal supporting arms. To preserve a true circular shape segments of wood are inserted between the arms, and the outside is turned to a cylindrical surface before the iron wire is coiled on to it. The outer edges of the arms are insulated with tape and fibre, so as to prevent contact between the arms, which are keyed to the spindle, and the core or the copper conductor. In Fig. 921a, a are the two sets of arms keyed to the spindle C, and having on each side projections e, by which stout fibre washers, d, are held in place. The object of these washers is to hold together lengthways the coils of iron wire (partly shown in the illustration) which form the armature core. After the same is completed, the wood segments are removed, and the copper conductor is wound over the core, and connected with a commutator in the usual Gramme method.

1 The author is indebted to the courtesy of the editor of "The Engineer" for this illustration, as well as for Figs. 86, 96, 97, 98, 101, 108.

The bars of the commutator are provided with long tags f, which serve to support a leather disc 7, by which means air is prevented from being drawn into the interior of the armature from the side of the commutator. Were this precaution not adopted, copper dust produced by the wearing of the brushes would be drawn by the current of air into the armature, and lodging between the internal coils, would sooner or later create a short circuit. The tags ƒ act as fan-blades, expelling the air at the left, and drawing it into the armature at the right, thus not only ventilating the internal coils, but also keeping the copper-dust out. The interior of the core itself is, however, not ventilated. The conductor is wound over the core in one layer only on the external surface in order that the distance between the core and pole pieces and, therefore, the magnetic resistance of the air space (see Chapter IV., page 108) may be a minimum.

The field magnets are formed by four slabs of wrought iron bolted together at the corners so as to form a rectangle, as will be seen from the perspective illustration, Fig. 93. In the longitudinal section (Fig. 92), S, S represent the cores proper of the magnets, and K, K are cast-iron pole pieces bolted on. To increase the magnetic conductivity a number of holes are drilled through the cast and wrought iron, and soft iron pins (shown in dotted lines) are tightly driven into these holes. In some of the later machines the pole pieces themselves are made of wrought iron, and then there is no need of this device. The field magnets are supported on a frame, M, of nonmagnetizable metal, which also serves as a base for the bearings of the armature spindle. This machine acts equally well as a dynamo or as a motor, but where economy of space is an object, as for instance in motors