The Siemens-Blondel oscillograph shown in [fig. 2,604], is of the moving coil type, being a development of the moving iron principle.
Moving Coil Oscillograph.—The operation of this form of oscillograph is based on the behaviour of a movable coil in a magnetic field.
Figs. 2,605 and 2,606.—Oscillograms reproduced from a paper by M. B. Field on "A Study of the Phenomena of Resonance by the Aid of Oscillograms" (Journal of E. E., Vol. XXXII). The effect of resonance on the wave forms of alternators has been the subject of much investigation and discussion; it is a matter of vital importance to the engineer in charge of a large alternating current power distribution system. Fig. 2,605 shows the pressure curve of an alternator running on a length of unloaded cable, the 11th harmonic being very prominent. Fig. 2,606 shows the striking alteration produced by reducing the length of cable in the circuit and thus causing resonance with the 13th harmonic.
It consists essentially of a modified moving coil galvanometer combined with a rotating or vibrating mirror, a moving photographic film, or a falling photographic plate. The galvanometer portion of the outfit is usually referred to as the oscillograph as illustrated in [figs. 2,608 to 2,612], representing diagrammatically the moving system.
In the narrow gap between the poles S, S of a powerful magnet are stretched two parallel conductors formed by bending a thin strip of phosphor bronze back on itself over an ivory pulley P. A spiral spring attached to this pulley serves to keep a uniform tension on the strips, and a guide piece L limits the length of the vibrating portion to the part actually in the magnetic field.
A small mirror M bridges across the two strips as shown. The effect of passing a current through such a "vibrator" is to cause one of the strips to advance while the other recedes, and the mirror is thus turned about a vertical axis.
