Scientific American Supplement, No. 421 | Page 2

an electrometric needle was of magnetized steel suspended by a cotton thread. In this instrument, a silver wire, t, terminating in a ball, is fixed to a support, C, hanging from a brass disk, P, placed upon the glass case of the apparatus. It will be seen that if we bring an electrified body near the disk, P, a deviation of the needle will occur. The sensitiveness of the latter may be regulated by a magnetic system like that of the galvanometer. Finally, a disk, P', which may be slid up and down its support, permits of the instrument being used as a condensing electrometer, by giving it, according to the distance of the disks, different degrees of sensitiveness. One constructor who furnished much to this part of the exhibition was Mr. Th. Edelmann of Munich, whose apparatus are represented in a group in Fig. 3. Among them we remark the following: A quadrant electrometer (Fig. 4), in which the horizontal 8-shaped needle is replaced by two connected cylindrical surfaces that move in a cylinder formed of four parts; a Von Beetz commutator; spyglasses with scale for reading measuring instruments (Fig. 3); apparatus for the study of magnetic variations, of Lamont (Fig. 3) and of Wild (Fig. 5); different types of the Wiedemann galvanometer; an electrometer for atmospheric observations (Fig. 6); a dropping apparatus (Fig. 7), in which the iron ball opens one current at a time at the moment it leaves the electro-magnet and when it reaches the foot of the support, these two breakages producing two induction sparks that exactly limit the length to be taken in order to measure the time upon the tracing of the chronoscope tuning-fork; an absolute galvanometer; a bifilar galvanometer (Fig. 8) for absolute measurements, in which the helix is carried by two vertical steel wires stretched from o to u, and which is rendered complete by a mirror for the reading, and a second and fixed helix, so that an electro-dynamometer may be made of it; and, finally, a galvanometer for strong currents, having a horseshoe magnet pivoted upon a vertically divided column which is traversed by the current, and a plug that may be arranged at different heights between the two parts of the column so as to render the apparatus more sensitive (Fig. 9).
[Illustration: FIG. 2.--ZENGER'S UNIVERSAL ELECTROMETER.]
We may likewise cite the exhibit of Mr. Eugene Hartmann of Wurtzburg, which comprised a series of apparatus of the same class as those that we have just enumerated--spyglasses for the reading of apparatus, galvanometers, magnetometers, etc.
[Illustration: FIG. 3.--EXHIBIT OF TH. EDELMANN.]
Specially worthy of remark were the apparatus of Mr. Kohlrausch for measuring resistances by means of induction currents, and a whole series of accessory instruments.
Among the objects shown by other exhibitors must be mentioned Prof. Von Waltenhofen's differential electromagnetic balance. In this, two iron cylinders are suspended from the extremities of a balance. One of them is of solid iron, and the other is of thin sheet iron and of larger diameter and is balanced by an additional weight. Both of them enter, up to their center, two solenoids. If a strong current be passed into these latter, the solid cylinder will be attracted; but if, on the contrary, the current be weak, the hollow cylinder will be attracted. If the change in the current's intensity occur gradually, there will be a moment in which the cylinders will remain in equilibrium.
[Illustration: FIG. 4.--EDELMANN'S QUADRANT ELECTROMETER.]
Prof. Zenger's differential photometer that we shall finally cite is an improvement upon Bunsen's. In the latter the position of the observer's eye not being fixed, the aspect of the spot changes accordingly, and errors are liable to result therefrom. Besides, because of the non-parallelism of the luminous rays, each of the two surfaces is not lighted equally, and hence again there may occur divergences. In order to avoid such inconveniences, Prof. Zenger gives his apparatus (Fig. 10) the following form: The screen, D, is contained in a cubical box capable of receiving, through apertures, light from sources placed upon the two rules, R and R'. A flaring tube, P, fixes the position of the eye very definitely. As for the screen, this is painted with black varnish, and three vertical windows, about an inch apart, are left in white upon its paper. Over one of the halves of these parts a solution of stearine is passed. To operate with the apparatus, in comparing two lights, the central spot is first brought to invisibility, and the distances of the sources are measured. A second determination is at once made by causing one of the two other spots to disappear, and the mean of the two results is then taken. As, at a maximum, there is a difference corresponding to 3/100 of a candle between the illumination of the