FIG. 142.

the generation of alternating currents is that designed by Mr. W. M. Mordey, and illustrated in fig. 142. There are in this

U

ture is fixed, and the field-magnet rotated; and, in the second place, the lines of force in the various fields projected through the armature coils are all in one direction, the pole-pieces on one side of the armature being all of north polarity, and those on the other side of south polarity. There is consequently no tendency to leakage between the adjacent pole-pieces, but the space between the opposing pole-faces is kept as small as practicable, not only in order to reduce the magnetic resistance, but also to prevent the spreading of the field, because it is necessary that any one armature coil shall at one moment embrace as many as possible, and at the next moment as few as possible, of the lines of force.

The armature, which is shown in fig. 143, consists of a number of coils of copper ribbon, wound on flat pear-shaped cores either of paraffined wood or porcelain, the different layers or turns of ribbon being insulated one from another by an insulating strip of the same width, which is wound in with them. The whole of the turns in each coil are bound together with strips of prepared tape. The outer or broad end of each coil is clamped by two short bolts between German-silver plates bearing against ebonite insulating sheets, and through these plates and the core of the coil is placed a third bolt, which also passes through a slotted hole in the internal flange of the frame, and so secures the coil in position. Each coil is also provided with a radial adjusting screw, one end bearing against the head of the coil and the other end against the inner face of the external flange; and when the third bolt mentioned above is loosened, the coil can be adjusted accurately in position by being urged radially inwards or outwards. The ends of adjacent coils are electrically connected by stiff conductors brought out through porcelain insulators as shown in the figure. The metal supports, which do not anywhere come between the poles of the field-magnets, are almost entirely beyond the magnetic field, and in this way the generation of wasteful eddy currents is reduced to a minimum, any slight loss that might arise from this cause being still further reduced by the employment of German silver for the brackets and bolts, as the high resistance of this alloy prevents the generation of any but exceedingly feeble currents. The gun-metal supporting ring, which is bolted to the bed-plate of the machine, is in two portions, being divided in a

vertical diametrical line. These two parts, after having received the coils, are bolted together and to the bed-plate, the fieldmagnets being, however, previously placed in position. By this device single coils, should they become faulty, can be easily and quickly removed for renewal or repair; or each half of the armature can be removed without difficulty by sliding it on to the stool shown in the figure.

The shape of the field-magnet is remarkable, and differs altogether from that of any other machine previously constructed.

It consists of a single electro-magnet built up in the following manner: A short cylinder of iron, through the axis of which the shaft passes, forms the core of the magnet, and is wound with a single exciting coil. To each end of this cylinder or core is attached a massive iron casting of peculiar form, which will be best under

stood from fig. 144. Each casting consists of a number of horns or claws, which bend over from their common junction, so that the extremities of the two sets of pole-pieces approach within a very short distance of each other, the narrow polar gap or slit thus formed being only just wide enough to contain the armature coils without touching them when the entire field-magnet is revolved. The ends of the 'exciting' coil (which is to be seen inside the pole-pieces in fig. 144) are connected to collector-rings on the shaft, as shown at c to the right of the figure. These might be dispensed with, and the exciting coil, as well as the armature, might be made stationary. The core and its pole pieces would then be the only portions revolving, and the electrical effect would be the same, but serious mechanical difficulties would arise in fixing the coil. Hence, it is far preferable to attach it to the rotating cylinder. The simplicity of this form of field-magnet is one of its great features, as a single exciting coil suffices for a machine of any size, speed, or number of alternations. The heavy rotating field-magnet acts very efficiently as a fly-wheel, and ensures steadiness in running, effectually neutralising, within certain wide limits, any pulsations due to irregularity in the stroke of the engine, and it is also advantageous when two or more machines are run so as to feed the same circuit in parallel. The insulation of the armature coils, as well as their connection with the external circuit, is also simplified, and being stationary they need only to be supported with a view to resisting the drag of the field, which is parallel to the plane of the coils. The armature is stiffened and held in its vertical position by substantial horizontal brackets bolted on to the bearing-standards.

The method of joining the armature coils together is similar in principle to that illustrated in fig. 141, the inner ends of the coils being joined together by short copper rods on one face of the frame, and the outer ends on the other face. The position of one of these sets of connecting wires or rods can be seen in fig. 143, although it may be mentioned that in some machines the whole of the connections are placed on one face of the armature. The number of alternations during each revolution is the same as in the case of a Siemens or a Ferranti machine having an equal number of armature coils, but there are only half as many pole

pieces as would be required for either of those machines. This arises from the fact that the current in each coil is reversed at that moment when it is between two opposite pole-faces, and again when it is midway between two adjacent pairs of pole-piecesthat is to say, at the moment when it has the maximum number of lines of force thrust through it, and again when it embraces the minimum number. It will be observed that in this machine the direction of the lines of force through the armature coils is

never reversed, the electro-motive force being produced by alternating the number of lines of force embraced by the coils between a maximum and a minimum.

Since the armature coils are similar as regards shape and the length of ribbon wound on them, and as the various fields projected through them are at any moment equal, the E.M.F. of any one coil is an aliquot part of the whole; and, as all the coils are fixed, the E.M.F. developed in any one coil can be easily measured,