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 horse