[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 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.