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Updated: May 9, 2025
It was one of those electrical spelling-advertisements, worked by a small motor commutator driven by a works-motor, and I had now set it going: for on some night before that Sabbath of doom the chemist must have set it to work, but finding the works abandoned, had not troubled to shut it down again.
This motor was made from the parts such as fields, armature, commutator, shaft and bearings, etc., of an Edison 'Z, or 60-light dynamo. It was the only size of dynamo that the Edison Company had marketed at that time.... As a motor, it was wound to run at maximum speed to develop a torque equal to about fifteen horse-power with 220 volts.
It was in his machine also that mica was used for the first time as an insulating medium in a commutator.
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.
By this plan currents of considerable intensity and alternating in direction at each revolution were induced in the coil. The ends of the coil were next connected to the external circuit through a "commutator."
To a student of to-day all this seems simple, but in those days the art of constructing dynamos was about as dark as air navigation is at present.... Edison also improved the armature by dividing it and the commutator into a far greater number of sections than up to that time had been the practice. He was also the first to use mica in insulating the commutator sections from each other."
We were actually running with an F.C. of 3.75 amps, for a period, when the sparking assumed the appearance of a ring of fire and, fearing a commutator strip would melt, I ordered an F.C. of five amps. We thus passed a quarter of an hour full of strain, the tension of which was reflected in the attitude of all the men.
The Edison machine is of this type, and is illustrated in figure 43, where M M' are the field magnets with their poles N S, between which the armature A is revolved by means of the belt B, and a pulley seen behind. The leading wires W W convey the current from the brushes of the commutator to the external circuit.
It was found necessary to push the armature along the spindle close to the commutator piece, C, and to shorten the spindle at the armature end and turn it down to the size of the original bearing, in order to bring the motor within the space between the wheels.
The commutator short-circuits the armature coils in succession in the proper positions to utilize the repulsive effect set up by the currents which are induced in them by the alternations in the field coils. The motor has no dead point, and will start from a state of rest and give out considerable power, but with what economy is not yet known.
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