The reading device of an analogue or digital measuring instrument is that part of the instrument designed to provide visual indications of measurements. The reading device of an analogue instrument usually consists of a scale and a pointer, either of which may be nonstationary. Depending
on the pointer, reading devices are either needle-type or light-beam-type. In needle-type devices the tip of the needle moves with respect to marks on a scale. The tip may be arrow-shaped or in the form of a knife or a taut thread (see Figure 1). In the last two cases, the scales are fitted with a mirror to eliminate reading errors caused by parallax. In light-beam-type reading devices a light beam reflected from a small mirror attached to the moving part of the instrument acts as the needle (see Figure 2).
The position of the light spot on the scale from which the readings are made depends on the position of the moving part. A light-beam-type reading device eliminates parallax error and increases instrument sensitivity by lengthening the pointer and doubling its angle of rotation.
The reading device of a digital instrument makes it possible to obtain a reading directly in digital form. Digital indicators of various designs are used to produce numerical images (see Figure 3). Mechanical indicators consist of several rollers or disks with numbers around the perimeter and a number of windows in which the digits appear (see Figure 3,a and 3,b). Electric-power meters, for example, are equipped with such reading devices.
Electromechanical indicators contain moving parts with digital images, with the parts moved by electromechanical driving mechanisms. Electrical indicators use incandescent lamps, luminescent and gas-discharge elements, and cathode-ray tubes to form images of numbers (see Figure 3,c, 3,d, and 3,e).
K. P. SHIROKOV