If a small soft iron or magnetic steel needle, _n
s_, be suspended between the poles, S N, of an electro magnet of such
proportions as described above, and the current, after exciting the
electro magnet, _e e_, be lead round the coils, DD, it will be found
that for all currents between 10 and 100 amperes the needle, _n
s_, shows a definite deflection for each current. Here we have a
galvanometer with permanent calibration. In this case the deflection of
the needle will not strictly follow the law of tangents, because the
directing power of the electro magnet is not absolutely constant; but
whatever the exact ratio between deflection and current may be, it must
always remain the same, and to each angle of deflection corresponds one
definite strength of current.
[Illustration]
The force with which the electro magnet tends to keep the needle in its
zero position, that is, in line with the poles, S N, is due partly to
the magnetism of the core, which is nearly constant, and partly to the
magnetic influence of the coils, _ee_, themselves, which is, of course,
simply proportional to the current.
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