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 two neighboring windows, in the given conditions of the apparatus, the error is thus limited to a half of this value, or 2 per cent. of that of one candle. [Illustration: FIG. 5.--WILD'S APPARATUS FOR STUDYING MAGNETIC VARIATIONS.] Among the apparatus designed for demonstration in lecture courses, we remarked a solenoid of Prof. Von Beetz for demonstrating the constitution of magnets (Fig. 11), and in which eight magnetized needles, carrying mica disks painted half white and half black, move under the influence of the currents that are traversing the solenoid, or of magnets that are bought near to it externally. Another apparatus of the same inventor is the lecture-course galvanometer (Fig. 3), in which the horizontal needle bends back vertically over the external surface of a cylinder that carries divisions that are plainly visible to spectators at a distance.