of copper, whose edges do not
meet, and which is connected by strips of copper with two terminals.
This internal shell is capable of serving for currents of quantity,
and, when the two terminals are united by a wire, it may serve as a
deadener. Above this copper shell there are two identical coils of
wire which may, according to circumstances, be coupled in tension or
in series, or be employed differentially. Reading is performed either
by the aid of a needle moving over a dial, or by means of a mirror,
which is not shown in the figure. Finally, there is a lateral scale,
R, which carries a magnetized bar, A, that may be slid toward the
galvanometer. This magnet is capable of rendering the needle less
sensitive or of making it astatic. In order to facilitate this
operation, the magnet carries at its extremity a tube which contains a
bar of soft iron that may be moved slightly so as to vary the length
of the magnet. Prof. Zenger calls this arrangement a magnetic vernier.
It will be seen that, upon combining all the elements of the
apparatus, we can obtain very different combinations; and, according
to the inventor, his rheometer is a substitute for a dozen
galvanometers of various degrees of sensitiveness, and permits of
measuring currents of from 20 amperes down to 1/50000000 an ampere.
The apparatus may even be employed for measuring magnetic forces, as
it constitutes a very sensitive magnetometer.

[Illustration: FIG. 1.--ZENGER'S UNIVERSAL RHEOMETER.]

Prof. Zenger likewise had on exhibition a "Universal Electrometer"
(Fig. 2), in which the fine wire that served as an electrometric
needle was of magnetized steel suspended by a cotton thread. In this
instrument, a silver wire, t, terminating in a ball, is fixed to a
support, C, hanging from a brass disk, P, placed upon the glass case
of the apparatus. It will be seen that if we bring an electrified body
near the disk, P, a deviation of the needle will occur. The
sensitiveness of the latter may be regulated by a magnetic system like
that of the galvanometer. Finally, a disk, P', which may be slid up
and down its support, permits of the instrument being used as a
condensing electrometer, by giving it, according to the distance of
the disks, different degrees of sensitiveness. One constructor who
furnished much to this part of the exhibition was Mr. Th. Edelmann of
Munich, whose apparatus are represented in a group in Fig. 3. Among
them we remark the following: A quadra