Curve, External Characteristic.
A characteristic curve of a dynamo, corresponding to the electro-motive
force curve, except that the ordinates represent the voltages of the
external circuit, the voltages as taken directly from the terminals of
the machine, instead of the total electro-motive force of the circuit.
The dynamo is run at constant speed. The resistance of the external
circuit is varied. The voltages at the terminals of the machine and the
amperages of current corresponding thereto are determined. Using the
voltages thus determined as ordinates and the corresponding amperages as
abscissas the external characteristic curve is plotted.
This curve can be mechanically produced. A pencil may be moved against a
constant force by two electro-magnets pulling at right angles to each
other. One must be excited by the main current of the machine, the other
by a shunt current from the terminals of the machine. The point of the
pencil will describe the curve.
Fig. 122. CHARACTERISTIC CURVE OF A DYNAMO.
Curve, Horse Power.
Curves indicating electric horse power. They are laid out with
co-ordinates, volts being laid off on the axis of ordinates, and amperes
on the axis of abscissas generally. The curves are drawn through points
where the product of amperes by volts equals 746. On the same diagram 1,
2, 3 .... and any other horse powers can be plotted if within the
limits. See Fig. 120.
Curve, Isochasmen.
A line drawn on the map of the earth's surface indicating the locus of
equal frequency of auroras.