Scientific American Supplement, No. 421, January 26, 1884 by Various


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Page 2

[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 quadrant electrometer (Fig. 4), in
which the horizontal 8-shaped needle is replaced by two connected
cylindrical surfaces that move in a cylinder formed of four parts; a
Von Beetz commutator; spyglasses with scale for reading measuring
instruments (Fig. 3); apparatus for the study of magnetic variations,
of Lamont (Fig. 3) and of Wild (Fig. 5); different types of the
Wiedemann galvanometer; an electrometer for atmospheric observations
(Fig. 6); a dropping apparatus (Fig. 7), in which the iron ball opens
one current at a time at the moment it leaves the electro-magnet and
when it reaches the foot of the support, these two breakages producing
two induction sparks that exactly limit the length to be taken in
order to measure the time upon the tracing of the chronoscope
tuning-fork; an absolute galvanometer; a bifilar galvanometer (Fig. 8)
for absolute measurements, in which the helix is carried by two
vertical steel wires stretched from o to u, and which is rendered
complete by a mirror for the reading, and a second and fixed helix, so
that an electro-dynamometer may be made of it; and, finally, a
galvanometer for strong currents, having a horseshoe magnet pivoted
upon a vertically divided column which is traversed by the current,
and a plug that may be arranged at different heights between the two
parts of the column so as to render the apparatus more sensitive (Fig.
9).

[Illustration: FIG. 2.--ZENGER'S UNIVERSAL ELECTROMETER.]

We may likewise cite the exhibit of Mr. Eugene Hartmann of Wurtzburg,
which comprised a series of apparatus of the same class as those that
we have just enumerated--spyglasses for the reading of apparatus,
galvanometers, magnetometers, etc.

[Illustration: FIG. 3.--EXHIBIT OF TH. EDELMANN.]

Specially worthy of remark were the apparatus of Mr. Kohlrausch for
measuring resistances by means of induction currents, and a whole
series of accessory instruments.

Among the objects shown by other exhibitors must be mentioned Prof.
Von Waltenhofen's differential electromagnetic balance. In this, two
iron cylinders are suspended from the extremities of a balance. One of
them is of solid iron, and the other is of thin sheet iron and of
larger diameter and is balanced by an additional weight. Both of them
enter, up to their center, two solenoids. If a strong current be
passed into these latter, the solid cylinder will be attracted; but
if, on the contrary, the current be weak, the hollow cylinder will be
attracted. If the change in the current's intensity occur gradually,
there will be a moment in which the cylinders will remain in
equilibrium.

[Illustration: FIG. 4.--EDELMANN'S QUADRANT ELECTROMETER.]

Prof. Zenger's differential photometer that we shall finally cite is
an improvement upon Bunsen's. In the latter the position of the
observer's eye not being fixed, the aspect of the spot changes
accordingly, and errors are liable to result therefrom. Besides,
because of the non-parallelism of the luminous rays, each of the two
surfaces is not lighted equally, and hence again there may occur
divergences. In order to avoid such inconveniences, Prof. Zenger gives
his apparatus (Fig. 10) the following form: The screen, D, is
contained in a cubical box capable of receiving, through apertures,
light from sources placed upon the two rules, R and R'. A flaring
tube, P, fixes the position of the eye very definitely. As for the
screen, this is painted with black varnish, and three vertical
windows, about an inch apart, are left in white upon its paper. Over
one of the halves of these parts a solution of stearine is passed. To
operate with the apparatus, in comparing two lights, the central spot
is first brought to invisibility, and the distances of the sources are
measured. A second determination is at once made by causing one of the
two other spots to disappear, and the mean of the two results is then
taken. As, at a maximum, there is a difference corresponding to 3/100
of a candle between the illumination of the two neighboring windows,
in the given conditions of the apparatus, the error is thus limited to
a half of this value, or 2 per cent. of that of one candle.

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