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Local clocks controlled from a standard clock by electricity have been more successful in practice, and are employed in several towns for example, Glasgow. Behind local dials are electromagnets which, by means of an armature working a frame and ratchet wheel, move the hands forward every minute or half-minute as the current is sent from the standard clock.
He dropped the great spoon and its armature of hooks over the side, and Brace glanced after it, to see it for a few moments as the line was allowed to run, the silvered unfishlike piece of metal beginning to spin and, as it receded farther from the boat, to assume a wonderfully lifelike resemblance to a good-sized roach swimming pretty fast.
At a distance where the current has become weak because of the resistance of the wire and losses due to faulty insulation, it will still operate a delicate electro-magnet with a very light armature so arranged as to open and close a local circuit provided with suitable batteries.
And was conscious of a deepened sense of security. Next going to the telephone, she removed the receiver from the hook and let it hang at the full length of the cord. In the dead silence the small voice of Central was clearly articulate: "What number? Hello, what number?" followed by the grumbling of the armature as the operator tried fruitlessly to ring the disconnected bell.
I have, however, extended the principle to the construction of more complete apparatus. One form has its revolving portion or armature composed of a number of sheet iron disks wound as usual with three coils crossing near the shaft. The commutator is arranged to short-circuit each of these coils in succession, and twice in a revolution, and for a period of 90-degrees of rotation each.
In this machine the conductors of the armature are not coils of wire, but separate bars of copper. In shunt machines the variation of current due to a varying number of lamps in use occasions a rise and fall in the brightness of the lamps which is undesirable, and hence a third class of dynamo has been devised, which combines the principles of both the series and shunt machines.
The armature is stationary, and the wires that start from the bobbins are connected with terminals placed upon a wooden support that surmounts the machine. This arrangement allows of every possible grouping of the currents according to requirements.
It consisted of an electro-magnet with double poles and an armature which pressed upon a disc or discs of plumbago, through which the local current Passed. The electro-magnet was excited by the main line current and the armature attracted to its poles at every signal, thus pressing on the plumbago, and by reducing its resistance varying the current in the local circuit.
There is plenty of scope for adjustment, as the wheels can be shifted in either direction longitudinally, while the distance between wheel and armature centres may be further modified in the length of the bearings, BE. These last are pieces of brass strip turned up at the ends, and bored for axles, and screwed to the under side of the base.
In other cases dynamos are used having collector rings for alternating current on one side and a commutator for direct current on the other side of the armature, thus enabling you, when the peak in two districts of a city comes at two different times, to take care of this peak by means of the same original generating unit, furnishing direct low-tension current to the points near the central station and alternating current to the distant points.
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