A curve indicating, graphically, the relations between any two factors,
which are interdependent, or which vary simultaneously. Thus in a
dynamo, the voltage increases with the speed of rotation, and a
characteristic curve may be based on the relations between the speed of
rotation and voltage developed. The current produced by a dynamo varies
with the electro-motive force, and a curve can express the relations
between the electro-motive force and the current produced.

A characteristic curve is usually laid out by rectangular co-ordinates
(see Co-ordinates). Two lines are drawn at right angles to each other,
one vertical, and the other horizontal. One set of data are marked off
on the horizontal line, say one ampere, two amperes, and so on, in the
case of a dynamo’s characteristic curve.

For each amperage of current there is a corresponding voltage in the
circuit. Therefore on each ampere mark a vertical is erected, and on
that the voltage corresponding to such amperage is laid off. This gives
a series of points, and these points may be connected by a curve. Such
curve will be a characteristic curve.

The more usual way of laying out a curve is to work directly upon the
two axes. On one is laid off the series of values of one set of data; on
the other the corresponding series of values of the other dependent
data. Vertical lines or ordinates, q. v., are erected on the horizontal
line or axis of abscissas at the points laid off; horizontal lines or
abscissas, q. v., are drawn from the points laid off on the vertical
line or axis of ordinates. The characteristic curve is determined by the
intersections of each corresponding pair of abscissa and ordinate.

Variations exist in characteristic curve methods. Thus to get the
characteristic of a commutator, radial lines may be drawn from a circle
representing its perimeter. Such lines may be of length proportional to
the voltage developed on the commutator at the points whence the lines
start. A cut giving an example of such a curve is given in Fig. 125.
(See Curve of Distribution of Potential in Armature.)

There is nothing absolute in the use of ordinates or abscissas. They may
be interchanged. Ordinarily voltages are laid off as ordinates, but the
practise may be reversed. The same liberty holds good for all
characteristic curves. Custom, however, should be followed.

Synonym–Characteristic.