DYNAMO-ELECTRIC MACHINES.—Since the date of the author's former paper in April, 1879, other observers have published the results of experiments similar to those described by him. It may be well to exhibit some of these results reduced to the form he has adopted, viz., a curve, such as that shown in Fig. 4, Proceedings, 1879, Plate 29, and now reproduced, with slight alterations, in Fig. 1. Here any abscissa represents a current passing through the dynamo-electric machine, and the corresponding ordinate represents the electromotive force of the machine for a certain speed of revolution, when that current is passing through it. It will be found (1) that with varying speed the ordinate or electromotive force, corresponding to any abscissa or current, is proportional to the speed; (2) that the electromotive force does not increase indefinitely with increasing current, but that the curve approaches an asymptote; (3) that the earlier part of the curve is, roughly speaking, a straight line, until the current attains a certain value, and that at that point the electromotive force has reached about two-thirds of its maximum value. When the current is such that the electromotive force is not more than two-thirds of its maximum, a very small change in the resistance with speed of engine constant, or in the speed of the engine with resistance constant, causes a great change in the current. For this reason such a current, which is the same for all speeds of revolution, since the curves for different speeds differ only in the scale of ordinates, may be called the “critical current” of the machine. The effect of a change of speed is exhibited in Fig. 1, where the lower line represents a curve for a speed of 660 revolutions per minute, instead of 720. The resistance, varying as (electromotive force)/current, is given by the slope of the line O P, which must therefore be constant; and it will be seen that this line cuts the upper curve at a point corresponding to a current of 15 webers, and the lower at a point corresponding to a current of 5 webers only.
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On Electric Lighting 1 . Nature 22, 42–44 (1880). https://doi.org/10.1038/022042a0