[Illustration: FIG. 6.--ELECTROMETER FOR ATMOSPHERIC OBSERVATIONS.]

Finally, let us cite an instrument designed for demonstrating the
principle of the Gramme machine. A circular magnet, AA', is inserted
into a bobbin, B, divided into two parts, and moves under the
influence of a disk, L, actuated by a winch, M. This system permits of
studying the currents developed in each portion of the bobbin during
the revolution of the ring (Fig. 12).

[Illustration: FIG. 7.--WIEDEMANN'S CURRENT BREAKER.]

To end our review of the scientific apparatus at the exhibition we
shall merely mention Mr. Van Rysselberghe's registering
thermometrograph (shown in Figs. 13 and 14), and shall then say a few
words concerning two types of registering apparatus--Mr. Harlacher's
water-current register and Prof. Von Beetz's chronograph.

[Illustration: FIG. 8.--WIEDEMANN'S BIFILAR GALVANOMETER.]

Mr. Harlacher's apparatus was devised by him for studying the deep
currents of the Elbe. It is carried (Fig. 15) by a long, vertical,
hollow rod which is plunged into the river. A cord that passes over a
pulley, P, allows of the apparatus, properly so called, being let down
to a certain depth in the water. What is registered is the velocity of
the vanes that are set in action by the current, and to effect such
registry each revolution of the helix produces in the box, C, an
electric contact that closes the circuit in the cable, F, attached to
the terminals, B. This cable forms part of a circuit that includes a
pile and a registering apparatus that is seen at L, outside of the box