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The continuous current and the alternating current generators invented by Dr. J. Hopkinson and Dr. Alexander Muirhead are peculiarly interesting as being probably the first in which the bobbins of the armature were wound with copper ribbon and arranged on a disk armature much in the same way as was afterward done by Sir William Thomson and by Mr. Ferranti. In the Muirhead-Hopkinson machine the armature coils are attached to a soft iron ring, whereas in the Ferranti the iron core is dispensed with, and a gain of lightness in the armature or rotating part effected; this advantage is of considerable importance, though Messrs. Hopkinson and Muirhead can of course reduce the weight of this iron core to insignificant proportions. [Illustration: HOPKINSON & MUIRHEAD'S DYNAMO-ELECTRIC GENERATOR.] The general form of this generator is clearly shown by the side and end elevation. The armature is made by taking a pulley and encircling it with a rim of sheet-iron bands, each insulated from the other by asbestos paper. On one or both sides of the rim thus formed, radial slots are cut to admit radial coils of insulated copper wire or ribbon, so that they lie in planes parallel to the plane of the pulley. In the continuous current machine coils are placed on both sides of the iron rim and arranged alternately, that on the one side always covering the gap between two on the other side. In this way, when a coil on one side of the rim is at its "dead point" and yields its minimum of current, the corresponding coil on the other side is giving out its maximum. The field magnets are made in a similar manner to the armature and run in circles parallel to the rim of the latter. The cores may be built up of wrought iron as the rim of the armature is; but it is found cheaper to make them of solid wrought or cast iron. To stop the local induced currents in the core, however, Messrs. Muirhead and Hopkinson cut grooves in the faces of the iron cores, and fill them up with sheet-iron strips insulated from each other, similar to the sheet-iron rim of the armature.