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.