spectators at a distance.

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).

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.

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.

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.

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.

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.

FIG. 12.—APPARATUS FOR DEMONSTRATING THE PRINCIPLE OF THE GRAMME MACHINE.

The apparatus is arranged in such a way that indications may thus be obtained upon the drum by means of induction sparks jumping between the style and the surface of the cylinder. To the left of the figure is seen the apparatus constructed by Lieutenant Ziegler for experimenting on the duration of combustion of bomb fuses.

FIG. 13.—VAN RYSSELBERGHE'S REGISTERING THERMOMETROGRAPH.

Shortly after the drum has commenced revolving, the contact, K, opens a current which supports the heavy armature, P, of an electro-magnet, M. This weight, P, falls upon the rod, d, and inflames the fuse, Z, at that very instant. At this precise moment the electro-magnet, H, inscribes a point, and renews it only when the cartridge at the extremity of the fuse explodes.

FIG. 14.—VAN RYSSELBERGHE'S REGISTERING THERMOMETROGRAPH.

This apparatus perhaps offers the inconvenience that the drum must be revolved by hand, and it would certainly be more convenient could it be put in movement at different velocities by means of a clockwork movement that would merely have to be thrown into gear at the desired moment. As it is, however, it presents valuable qualities, and, although it has already been employed in Germany for some time, it will be called upon to render still more extensive services.

FIG. 15.—HARLACHER'S APPARATUS FOR STUDYING DEEP CURRENTS IN RIVERS.

We have now exhausted the subject of the apparatus of precision that were comprised in the Munich Exhibition. In general, it may be said that this class of instruments was very well represented there as regards numbers, and, on another hand, the manufacturers are to be congratulated for the care bestowed on their construction.—La Lumiere Electrique.

FIG. 16.—HARLACHER'S APPARATUS FOR STUDYING DEEP CURRENTS IN RIVERS.

FIG. 17.—VON BEETZ'S CHRONOGRAPH.

COPPER VOLTAMETER.

Dr. Hammerl, of the Vienna Academy of Sciences, has made some experiments upon the disturbing influences on the correct indications of a copper voltameter. He investigated the effects of the intensity of the current, the distance apart of the plates, and their preparation before weighing. The main conclusion which he arrives at is this: That in order that the deposit should be proportional to the intensity of the current, the latter ought not to exceed seven ampères per square decimeter of area of the cathode.

Speaking of steel ropes as transmitters of