Page 79 - Teach Yourself Electricity and Electronics

Other specialized meter types 59

Sometimes, meters such as the one in Fig. 3-14 are used to measure infrared or ul-
traviolet intensity. Different types of photovoltaic cells have peak sensitivity at different
wavelengths. Filters can be used to block out wavelengths that you don’t want the me-
ter to detect.

Pen recorders
A meter movement can be equipped with a marking device, usually a pen, to keep a
graphic record of the level of some quantity with respect to time. Such a device is called
a pen recorder. The paper, with a calibrated scale, is taped to a rotating drum. The
drum, driven by a clock motor, turns at a slow rate, such as one revolution per hour or
one revolution in 24 hours. A simplified drawing of a pen recorder is shown in Fig. 3-15.

3-15 Simplified drawing of a pen recorder.

A device of this kind, along with a wattmeter, might be employed to get a reading
of the power consumed by your household at various times during the day. In this way
you might tell when you use the most power, and at what particular times you might be
using too much.
Oscilloscopes
Another graphic meter is the oscilloscope. This measures and records quantities that
vary rapidly, at rates of hundreds, thousands, or millions of times per second. It creates
a “graph” by throwing a beam of electrons at a phosphor screen. A cathode-ray tube,
similar to the kind in a television set, is employed.
Oscilloscopes are useful for looking at the shapes of signal waveforms, and also for
measuring peak signal levels (rather than just the effective levels). An oscilloscope can
also be used to approximately measure the frequency of a waveform. The horizontal
scale of an oscilloscope shows time, and the vertical scale shows instantaneous voltage.
An oscilloscope can indirectly measure power or current, by using a known value of re-
sistance across the input terminals.