Fig. 1 shows an elevation of the instrument and a horizontal section of
the bobbins. Two pairs of bobbins, cc, cc, are so arranged that the axes
of each pair are parallel and in the same vertical plane. Each pair is
supported by a vertical brass plate, and the two plates make an angle of
about 106° with each other, so that the planes containing the axes of the
bobbins make an angle of about 74°. Two horseshoe magnets, m m, made of
1/25 inch steel wire, are connected by a very light piece of aluminum and
placed at such a distance from each other that, on being suspended, the
two branches of each of the magnets shall freely enter the respective
bores of the two bobbins fixed upon the same plate, and, when the whole
system is in equilibrium and the bobbins free from current, the two
branches of each of the magnets shall nearly coincide with the axes of
such bores. The magnets are not plane, but are curved so as to form
portions of a vertical cylinder whose axis coincides with the direction
of the suspension wire, and to which the axes of the bobbins are tangent
at their center, approximately to the points where the poles of the
magnets are situated.
[Illustration: FIG. 1. GRAY'S GALVANOMETER.]
The needles have been given this form so that their extremities shall not
touch the sides of the bore during considerable deflections.
In the instrument which the inventors, Messrs. T. & A. Gray, used in
their experiments upon the resistance of glass, the needles were arranged
so that their poles of contrary name were opposite.
[Illustration: FIG. 2.]
The system of needles is suspended from the extremity of a screw, p,
which passes into a nut, n, movable between two stationary pieces. On
revolving the nut, we cause the screw to rise or lower, along with the
entire suspended part, without twisting the thread.
The four bobbins are grouped for tension, and have a total resistance of
30,220 ohms. They contain 16,000 feet of No. 50 copper wire, forming
62,939 revolutions, nearly equally divided between the four bobbins. When
a current is passing through the bobbins, the poles of one of the
horseshoe magnets are attracted toward the interior of the corresponding
bobbins, while those of the other are repelled toward the exterior by the
two other bobbins. We thus have a couple which tends to cause the system
to revolve around the suspension axis. A mirror, which is fixed upon a
vertical piece of aluminum, a, gives, in the usual manner, a reflected
image upon a scale, thus allowing the deflections to be read. A
compensating magnet, M, is supported by a vertical column fixed to the
case, above the needles. This magnet may be placed in the different
azimuths by means of a tangential screw, t. The extremities of the bobbin
wires are connected with three terminals, T, T¹, T², and the
instrument may, by a proper arrangement, became differential. These
terminals, as well as the communicating wires, are insulated with
ebonite.
Thus arranged, the instrument is capable of making a deflection of one
division of 1/50 inch upon a scale placed at a distance of a little more
than a yard, with the current produced by one daniell of 10 ohms. This is
a degree of sensitiveness that cannot be obtained with any of the astatic
instruments known up to the present. By regulating the needles properly,
a greater degree of sensitiveness may be attained, but then the duration
of the needles' oscillation becomes too great. The sensitiveness of the
instrument is sufficiently great to allow it to be used in many cases,
even with a moderate duration of oscillation.
In their experiments upon the resistance of glass, the inventors employed
an instrument that was not arranged for giving great sensitiveness, and
one with which resistances of from 10^{4} to 10^{5} megohms could be
measured by the use of a pile of 120 daniells.
The instrument can be given another form. The four bobbins may be
arranged symmetrically in the same plane, and the two horseshoe magnets
be supported by an S-shaped aluminum bar. The latter traverses the plate
that supports the bobbins, in such a way that one of the magnets enters
one of the bobbins that correspond to it on one side of the plate, and
the other on the other side, as shown in Fig. 2. The bobbins are so
connected that, when they are traversed by a current, both magnets are at
the same time attracted toward the interior or repelled toward the
exterior of the bobbins. Such a form of the instrument has the advantage
of being more easily constructed, while the regulation of the magnets
with respect to the bore of the bobbins is easier.
The chief advantage of the instrument results from the fact that, owing
to the arrangement of the magnets and bobbins, a large portion of the
wires of the latter is situated very near the poles of the magnets, and
in a position very favorable for electro-magnetic action. The instrument
presents no difficulties as regards construction, and costs no more than
an ordinary one.
We might even arrange a single horseshoe magnet, or an S-shaped one,
horizontally, and employ but a single pair of bobbins, and thus have a
non-astatic apparatus based upon the same principle. But in astatic
instruments it is better to place the magnets in such a way that the two
branches shall be in the same vertical plane.
Were the line that joins the two poles vertical, the system would be
perfectly astatic in a uniform field, since each magnet in particular
would then be perfectly astatic. A pair of horseshoe magnets may thus be
regulated in such a way as to form a perfectly astatic system in a
uniform field and to preserve an almost invariable zero, this being
something that it is very difficult to obtain with the ordinary
arrangement of needles, especially when a compensating magnet is used;
for, in such a case, one of the needles becomes more or less magnetized,
while the other becomes demagnetized, according to the position of the
compensating magnet.--_La Lumiere Electrique_.
* * * * *