An excellent simple device, known at, the " Ondo scope," has been devised by Prof. Ruhmer, of Berlin for examining alternating or pulsating electric dis charges. The following description is taken from the "Electrical Review", London:

The principle of the apparatus is based upon the the discovery by H. A. Wilson, in 1902, of the fact that the violet glow round the cathode of a Geissler tube varies in dimensions in proportion to the current strength, while it is, of course, free from inertia and, therefore, follows changes in the current with absolute precision and instantaneity. The instrument devised by Ruhmer to take advantage of this phenomenon consists of nothing more than two wires sealed into an exhausted tube about a foot in length and one inch in diameter, with their inner ends close to a small hole in a mica screen.

Viewed directly, when it is connected with the terminals of an induction coil, a steady violet glow surrounds the end of each wire; but when viewed in a revolving mirror, it is seen that the glow occurs alternately on the one and the other of the wires, never on both at once. Moreover, the luminous images, drawn out by the inventor of the mirror, show by their outline the changes in the current strength with regard to time and in its direction. The effect of varying the frequency and duration of make and break, the potential difference applied to the primary coil and the nature of the secondary circuit can thus be studied with the greatest ease and convenience.

On inserting a Villard valve-tube in the high-pressure circuit, the current becomes unidirectional, though of course still intermittent. Using the ondo-scope in a series with an X-ray tube, with a valve-tube in circuit, the strength of current can be estimated, or even roughly read off on a scale, with the instrument; the latter also serves to verify the fact that all the discharges occur in one direction only, testifying to the efficacy of the valve-tube which exercises an extraordinary powerful action as a rectifier. A singular fact which is shown by the ondoscope is that the discharge through an X-ray tube is practically instantaneous, the luminous image consisting simply of a narrow streak, no matter how rapidly the mirror was turned. By photographing the images a permanent record can, of course, be obtained.

The ondoscope with about 300 volts potential difference, so that it is available for use on ordinary high-pressure alternating circuits; while it is lacking in some of the advantages of the oscillograph, it unquestionably yields a considerable amount of information regarding wave-form, oscillations, etc., with the minimum of trouble and expense, and it can hardly get out of order.