Figs. 2,618 and 2,619.—Two curves obtained with the falling plate camera and illustrating the discharge of a condenser through an inductive circuit. When taking curve A the resistance in the circuit was very small compared to the inductance, while before taking curve B an additional non-inductive resistance was inserted in the circuit so that the oscillations were damped out much more rapidly although the periodic time remained approximately constant.
Ques. How is the time scale produced?
Ans. Either the surface on which the beam of light falls may be caused to move in a vertical plane with a certain velocity, so that the intersection of the beam and the plane surface traces out a curve connecting current with time (a curve which becomes a permanent record if a sensitized surface be used); or, the surface may remain stationary and in the path of the horizontally vibrating beam may be introduced a mirror which rotates or vibrates about a horizontal axis, thus superposing a vertical motion proportional to time on the horizontal vibration which is proportional to current, and causing the beam of light to trace out a curve connecting current and time on the stationary surface.
Ques. What kind of recording apparatus is used with the Duddell oscillograph?
Ans. A falling plate camera, or a cinematograph film camera.
Fig. 2,620.—Synchronous motor with vibrating mirror as used with Duddell moving coil oscillograph. Since the motor must run synchronously with the wave form it is required to investigate, it should be supplied with current from the same source. The motor can be used over a wide range of frequencies (from 20 to 120). When working at frequencies below 40, it is advisable to increase the moment of inertia of the armature, and for this purpose a suitable brass disc is used. The armature carries a sector, which cuts off the light from the arc lamp during a fraction of each revolution, and a cam which rocks the vibrating mirror. It makes one revolution during two complete periods, and the cam and sector are so arranged that during 1½ periods, the mirror is turning with uniform angular velocity, while during the remaining half period, the mirror is brought back quickly to its angular position, the light being cut off by the sector during this half period.
Ques. Explain the operation of the falling plate camera.
Ans. In this arrangement a photographic plate is allowed to fall freely by the force of gravity down a dark slide. At a certain point in its fall it passes a horizontal slit through which the beams of light from the oscillograph pass, tracing out the curves on the plate as it falls.