A characteristic curve of an electromagnet, indicating the relation of
magnetization to exciting current. Laying off on the axis of ordinates
the quantities of magnetism evoked, and the corresponding strengths of
the exciting current on the axis of abscissas, the curve can be plotted.
It first rises rapidly, indicating a rapid increase of magnetization,
but grows nearly horizontal as the iron becomes more saturated. The
effect due to the coils alone, or the effect produced in the absence of
iron is a straight line, because air does not change in permeability.
Curve of Distribution of Potential in
A characteristic curve indicating the distribution of potential
difference between adjoining sections of the commutator of an armature
in different positions all around it. The potential differences are
taken by a volt-meter or potential galvanometer, connection with the
armature being made by two small metal brushes, held at a distance apart
equal to the distance from centre to centre of two adjoining commutator
bars. The curve is laid out as if by polar co-ordinates extending around
the cross-section of the commutator, with the distances from the
commutator surface to the curve proportional to the potential
differences as determined by shifting the pair of brushes all around the
The above is S. P. Thompson's method. Another method of W. M. Mordey
involves the use of a pilot brush. (See Brush, Pilot.) Otherwise the
method is in general terms identical with the above.
Fig. 124. MAGNETIZATION CURVE.
Fig. 125. ARMATURE: CURVE.
Fig. 126. DEVELOPMENT OF ARMATURE CURVE.
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