Another graphic metering device 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. Some oscilloscopes have electronic conversion circuits that allow for the use of a solid-state liquid crystal display (LCD).

Oscilloscopes are useful for observing and analyzing 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 the instantaneous signal voltage. An oscilloscope can indirectly measure power or current, by using a known value of resistance across the input terminals.

Technicians and engineers develop a sense of what a signal waveform should look like, and then they can often tell, by observing the oscilloscope display, whether or not the circuit under test is behaving the way it should. This is a subjective measurement, because it is qualitative as well as quantitative.