in which it is usually inclosed. In certain cases, a bell whose sound
indicates the velocity of the current to the ear is substituted for
the registering apparatus.

[Illustration: FIG. 9.--WIEDEMANN'S GALVANOMETER FOR STRONG CURRENTS.]

Fig. 16 represents another type of the same apparatus in which the
mechanism of the contact is uncovered. The supporting rod is likewise
in this type utilized as a current conductor.

[Illustration: FIG. 10.--ZENGER'S DIFFERENTIAL PHOTOMETER.]

It now remains to say a few words about Prof. Von Beetz's chronograph.
This instrument (Fig. 17) is designed for determining the duration of
combustion of different powders, the velocity of projectiles, etc. The
registering drum, T, is revolved by hand through a winch, L, and the
time is inscribed thereon by an electric tuning fork, S, set in motion
by the large electro-magnet, E F. Each undulation of the curves
corresponds to a hundredth of a second. The tuning-fork and the
registering electro-magnets, G and H, are placed upon a regulatable
support, C, by means of which they may be given any position desired.

[Illustration: FIG. 11.--VON BEETZ'S SOLENOID FOR DEMONSTRATING THE
CONSTITUTION OF MAGNETS.]

The style, c, of the magnet, C, traces a point every second in order
to facilitate the reading. The style, b, of the electro-magnet, H,
registers the beginning and end of the phenomena that are being
studied.