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.

[Illustration:  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.

[Illustration:  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.

[Illustration:  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.

[Illustration:  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.

[Illustration:  Fig. 16.—­Harlacher’s apparatus for studying deep currents in Rivers.]

[Illustration:  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 amperes per square decimeter of area of the cathode.