The Oscillograph

Author: E.E. Kimberly

Because of the inertia of the moving parts of the more common types of electrical measuring instruments, their indications cannot follow accurately any rapid fluctuations of torque. Hence, they can only show readings proportional to average torque, and so are of no value in determining instantaneous values.

Fig. 16-18. Typical Oscillogram

The Duddell type of oscillograph operates on the principle of the D'Arsonval-type voltmeter, but the weight of the moving element is so small that its time lag because of inertia is negligible. In order to reduce the weight, the coil is made of only one loop of very small span. A mirror cemented to the coil provides an optic lever by which the coil deflection is indicated on a viewing screen. Between the mirror and the viewing screen is an oscillating or rotating mirror which deflects the beam at an angle to the direction imparted to it by the mirror. The result of the two motions of the light beam is a pattern in two dimensions such as that in Fig. 16-18, which shows the wave form of the exciting current of a transformer. Such instruments are useful in determining wave forms of currents or voltages and the transient starting current inrushes of motors, transformers, etc.

Another type of oscillograph uses a cathode-ray electron tube, and the two-dimensional image is made to appear on the flattened end of the tube. This type of instrument is especially useful in the analysis of ultrahigh speed transients to which the Duddell type of instrument is too sluggish to respond. The electromagnetic or Duddell type of oscillograph is useful for studying phenomena which require 50 micro-seconds or more. For phenomena of shorter duration the cathode-ray type of instrument is required. Both types of oscillograph are adapted to visual uses and to the making of permanent photographic records.