# Measurements

Instruments can be classified as either analogue or digital. Many analogue instruments are often termed (incorrectly) meters. Strictly speaking, a meter is a summating device, e.g. the kilowatt-hour meter.

Analogue instruments possess a deflecting device, a controlling device and a damping device. The sensitivity of such instruments is given by

A shunt is a non-inductive resistor connected across an instrument to extend its current range. A multiplier is a non-inductive resistor connected in series with an instrument to enable it to read voltage.

The scale of a moving coil instrument is linear and the delicate nature of the moving system limits the current to be measured to a low value, without the aid of a shunt.

Thermocouple instruments work on the principle of reacting to the temperature of a fine wire when a current is passed through it. They read true r.m.s. values irrespective of waveform, but possess a non-linear scale, low overload capacity, and are sluggish in response.

Electronic voltmeters can be very accurate and have a high input impedance and high sensitivity.

The absorption frequency meter and dip meter (dip oscillator) are used as guides only for frequency measurement. The digital frequency meter can give an accurate reading over a wide range and is reasonably cheap.

An oscilloscope is basically a measuring instrument, but its versatility enables it not only to make measurements but to display, essentially, graphs of voltage against time. The voltage under investigation is applied, via the Y-amplifier, to the Y-plates. The variable timebase, applied to the X-plates, allows the ‘spot’ to move horizontally at different fixed speeds so that a graph of voltage against time is produced.

With full carrier amplitude modulation the oscilloscope can be used to check the percentage modulation, which must of course not exceed 100%. When used in conjunction with a two-tone AF oscillator, the oscilloscope can indicate the power of a single sideband transmission.

All measuring instruments are subject to error. If the accuracy of the measuring instruments is known, a tolerance can then be given to all measurements.

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