We give herewith, from the _Elektrotechnische Zeitschrift_, a few
interesting details in regard to the measuring apparatus of Messrs.
Kapp and Crompton.
It is evident that when we use permanent magnets or springs as
directing forces in measuring instruments, we cannot count upon an
absolute constancy in the indications, as the magnetism of the
magnetized pieces, or the tension of the springs, modifies in time.
The apparatus require to be regulated from time to time, and hence the
idea of substituting electro-magnets for permanent ones.
[Illustration: FIG. 1.]
If we suppose (Fig. 1) a magnetized needle, _n s_, placed between the
extremities of a soft iron core, N S, and if we group the circuit in
such a way that the current, after traversing the coil, _e e_, of the
electro, traverses a circle, _d d_, situated in a plane at right
angles with the plane of the needle's oscillation, it is evident that
we shall have obtained an apparatus that satisfies the aforesaid
conditions. It seems at first sight that in such an instrument the
directing force should be constant from the moment the electro was
saturated, and it would be possible, were sufficiently thin cores
used, to obtain a constancy in the directing magnetic field for
relatively feeble intensities. In reality, the actions are more
complex. The needle, _n s_, is, in fact, induced to return to its
position of equilibrium by two forces, the first of which (the
attraction of the poles, N S) rapidly increases with the intensity so
as to become quickly and perceptibly constant, while the second (the
sum of the elementary electrodynamic actions that are exerted between
the spirals, _e e_, and the needle, _n s_) increases proportionally to
the intensity of the current. If we represent these two sections
graphically by referring the magnetic moments as ordinates and the
current intensities as abscissas to two co-ordinate axes (Fig. 2), we
shall obtain for the first force the curve, O A B, which, starting
from A, becomes sensibly parallel with the axis of X, and for the
second the right line, O D. The resultant action is represented by the
curve, O E E'_F. It will be seen that this action, far from being
constant, increases quite rapidly with the intensity of the current,
so that the deflections would become feebler and feebler for strong
intensities, of current; and this, as well known, would render the
apparatus very defective from a practical point of view.
[Illustration: FIG. 2.]
But the action of the spirals can be annulled without sensibly
diminishing the magnetism of the core by arranging a second system of
spirals identical with the first, but placed in a plane at right
angles therewith, or, more simply still, by having a single system of
spirals comprising the coil of the electro-magnet, but distributed in
a plane that is oblique with respect to the needle's position of rest.
It then becomes possible, by properly modifying such angle of
inclination, to obtain a total directing action that shall continue to
increase with the intensity, and which, graphically represented, shall
give the curve, O G G'_H, for example (Fig. 2).
[Illustration: FIG. 3.]
[Illustration: FIG. 4.]
[Illustration: FIG. 5.]
This arrangement, which is adopted in Mr. Kapp's instruments, gives
very good results, as may be easily seen by reference to Figs. 3 and
4, in which the current intensities or differences of potential are
referred as ordinates and the degrees of deflection of the needle as
abscissas. The unbroken lines represent the curves obtained with the
apparatus just described, while the dotted ones give the curve of
deflection of an ordinary tangent galvanometer. These curves show that
for strong intensities of current Mr. Kapp's instrument is more
advantageous than the tangent galvanometer. Mr. Crompton has
constructed an amperemeter upon the same principle, which is shown in
Fig. 5.--_La Lumiere Electrique._
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