The total magnetic force acting on
the needle is, therefore, represented by the sum of these two forces,
and consequently not nearly so constant as might be desired in order to
get a good imitation of a tangent galvanometer with a permanent magnet.
In the diagram, Fig. 2, the curve, O A B, represents the magnetic moment
of the iron core, the straight line, ODE, that of the exciting coils per
se, and the dotted line, O F M, the sum of the two, obtained by adding
for every current, O C, the respective ordinates, CD and C A.
CF = CD + CA
The rise of this curve shows that the force which tends to bring the
needle back to its zero position increases with the current, though at a
slower ratio than the deflecting force of the current. It follows from
this that for large currents the increment in the angle of deflection is
comparatively small, and the divisions on the scale whereon the current
is to be read off would come too near together to allow accurate
readings to be taken. In other words, the range of accurate reading in
an instrument so constructed would only be limited.
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