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 of voltage which are performed rapidly during each period by the alternating current. The voltmeter needle may then be made to record its variations graphically on a drum covered with paper and so to delineate the wave form of the current. The process is analogous to the optical experiment of looking at a quickly rotating wheel or engine through slits in a disk, rotating slightly faster or slower than the object observed. We then see the engine going through all its motions but much more slowly, and can follow them easily. In another form devised by Callendar, a revolving contact disk is placed on the shaft of an alternator, or of a synchronous motor driven by the alternating current under test. A pair of contact springs are slowly shifted over so as to close the circuit at successive assigned instants during a complete phase. The electromotive force so selected is balanced against the steady potential difference produced between a fixed and a sliding contact on a wire traversed by another steady current, and if there is any difference between this last, the potential difference, and the instantaneous potential difference balanced against it, a relay is operated and sets in action a motor which shifts the contact point along the potentiometer wire and so restores the balance. This contact point also carries a pen which moves over a rotating drum covered with paper. As the brushes are slowly shifted over on the revolving contact so as to select different phases of the alternating electromotive force, the pen follows and draws a curve delineating the wave form of that electromotive force or current. An instrument devised by E. B. Rosa is not very different in construction. A commutator method has also been devised by T. R. Lyle (Phil. Mag., November 1903, 6. 517) in which at an assigned instant during the phase a selection is made from the periodic current and measured on a galvanometer.

The oscillographs of A. E. Blondel and W. Duddell operate on a different principle. They consist essentially of a galvanometer of which the needle or coil has such a short natural periodic time that it can follow all the variations of a current which runs through its cycle in say th second. This needle or coil must be so damped that when the current is cut off it returns to zero at once without overshooting the mark. By means of an attached mirror and reflected ray of light the motion of the movable system can be indicated on a screen. This ray is also given a periodic motion of the same frequency by reflection from a separate oscillating mirror so as to make the two motions at right angles to one another, and thus we have depicted on the screen a bright line having the same form as the periodic current being tested. In W. Duddell’s oscillograph (fig. 1) the galvanometer part consists of an electromagnet in the field of which is stretched a loop of very fine wire. To this is attached a mirror; hence, if a current goes up one side of a loop and down another, the wires are oppositely displaced in the field. The loop and mirror move in a cavity full of oil to render the system dead-beat. A ray of light is reflected from this mirror and from another mirror which is rocked by a small motor driven off the same circuit, so that the ray has two vibratory motions imparted to it at right angles, one a simple harmonic motion and the other a motion imitating the variation of the current or electromotive force under test. This ray can be received on a screen or photographic plate, and thus the wave form of the current is recorded. In the Duddell oscillograph it is usual to place a pair of loops in the magnetic field, each with its own mirror, so that a pair of curves can be delineated at the same time, and if there is any difference in phase between them, it will be detected. Thus we can take two curves, one showing the potential difference at the end of an inductive circuit, and the other the current flowing through the circuit. In one form of Blondel’s oscillograph, the vibrating system is a small magnetic needle carrying a mirror, but the principle on which it operates is the same as that of the instrument above described. The oscillograph can be made to exhibit optically the form of the current curve in non-cyclical phenomena, such as the discharge of a condenser. In this case the large vibrating mirror must be oscillated by a current from an alternator, on the shaft of which is a disk of nonconducting material with brass slips let into it and so arranged with contact brushes that in each period of the alternator a contact is made, charging say a condenser and discharging it through the oscillograph. In this way an optical representation is obtained of the oscillatory discharge of the condenser. A form of thermal oscillograph has been devised by J. T. Irwin (Jour. Inst. Elec. Eng. Lond. 1907. 39. 617). In this instrument the periodic current, the time variation of which is being studied, passes through a pair of fine wires or strips, going up one wire and down the other. These wires are also traversed in the same direction by a constant current from a battery. The two currents are therefore added in one wire and subtracted in the other, and produce a differential heating effect which causes unequal expansion, and this in turn is made to tilt a   mirror which reflects a ray of light on to a screen or photographic plate as in the Duddell oscillograph.

Finally, purely optical methods have been employed. Braun devised a form of cathode ray tube, consisting of a vacuum tube having a narrow tubular portion and a bulbous end. The cathode terminal is connected to the negative pole of an electrostatic machine, such as a Wimshurst or Voss machine, giving a steady pressure. A cathode discharge is projected through two small holes in plates in the narrow part of the tube on a fluorescent screen at the end of the enlarged end, and the cathode ray or pencil depicts on it a small bright greenish patch of light. If a pair of coils of wire through which an alternating current is passing are placed on either side of the tube, just beyond one of the plates with a hole in it, the field causes a periodic displacement of the cathode ray and elongates the patch of light into a bright line. If this patch is also given a displacement in the direction of right angles by examining it in a steadily vibrating mirror, we see a wavy or oscillatory line of light which is an optical representation of the wave form of a current in the coils embracing the Braun tube.



.—See J. A. Fleming, A Handbook for the Electrical Laboratory and Testing Room, vol. i. (London, 1901), which contains a list of original papers on the oscillograph; Id., The Principles of Electric Wave Telegraphy (London, 1906), which gives illustrations of the use of the oscillograph and the Braun cathode ray tube in depicting condenser discharges; also, for the development of the oscillograph, A. E. Blondel, “Oscillographs: New Apparatus for registering Electrical Oscillations” (a short description of the bifilar and soft iron oscillographs), Comptes rendus (1893), 116. 502; Id., “On the Determination and Photographic Registration of Periodic Curves,” La Lumière électrique (August 29th, 1901); Id.,