) But such a system would appear to be too
complicated for ready use. Moreover, some sort of indicator would be
required by which we could make sure that the exciting current has the
normal strength.
[Illustration: FIG 4.]
The plan we adopt is to excite the electro magnet by the whole or a part
of the current which is to be measured. Since this current varies, the
power exciting the core of the electro magnet must also vary; and since
we require the core to have as nearly as possible a permanent magnetic
force, we are brought face to face with the question, whether an electro
magnet can be constructed that has a constant moment under varying
exciting currents. This question has been answered by the well known
experiments of Jacobi, Dub, Mueller, Weber, and others. To get an
absolutely constant magnetic moment, is not possible, but between
certain limits we can get a very near approximation to constancy.
[Illustration]
The relation between exciting power and magnetic moment is very
complicated, depending not only on the dimensions and shape of the core
and the manner of winding, but also on the chemical constitution of the
iron of the core.
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