machines are generally made with four or more poles.
Another feature of modern machines is the secure fastening of the active wires. Fig. 17 is merely a diagrammatic representation to illustrate a principle; it is not a drawing of a real machine. If the wire were simply wound over the outside of a smooth drum it would be very difficult to hold it securely in place. Not only would the wire bulge out
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by reason of centrifugal force, but the magnetic drag on the wire would displace it along the circumference. In modern machines the wires or coil sides, as the case may be, are secured in position by being placed in slots, as shown in Fig. 18. The core of the armature consists of thin iron plates which are slotted on special stamping machines. In building up the core, the plates are so laid one upon the other that all the slots register properly and form grooves parallel to the axis, into which’ the wires are
