In this way it is possible to obtain a more evenly divided scale than
in the case when the deflection follows the law of tangents, as in an
ordinary tangent galvanometer. This principle of overbalancing the
exciting coils is shown on diagram, Fig. 2. The straight line, O G,
represents the magnetic effect on the needle of that component of the
deflecting force which is parallel, but in sense opposed to S N;
as mentioned above, the magnetic effect of the exciting coils is
represented by the straight line, O E. The combined effect of these two
forces on the needle is represented by the line, O K, the ordinates
of which must be deducted from those of the curve, O A B, in order to
obtain the total directing force due to each current. This is shown by
the curve, O P Q, shown in a thick full line. This curve shows how
the directing force or strength of field in which the needle swings
decreases with an increasing current. That this does actually take place
can easily be proved by experiment.
Fig. 4 shows two curves; the one drawn in a full line is obtained
by plotting the deflection in degrees of the needle of a potential
indicator as abscissae, and the corresponding electromotive forces
measured simultaneously on a standard instrument as ordinates; the
dotted line shows what this curve would be with an ordinary tangent
galvanometer.
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