Oscilloscopes:

The oscilloscope is the graphic meter. This measures and records quantity which vary rapidly, at rates of hundreds, thousands, and millions of times per second. It creates a graph by throwing a beam of electrons at the phosphor screen. A cathode-ray tube, which is similar to the kind in a TV, is employed.

Oscilloscopes are very useful for looking at shapes of signal waveforms, and for measuring peak signal level also. An oscilloscope can be used to measure the frequency of a waveform. The horizontal scale of the oscilloscope shows time, and the vertical scale shows the instantaneous voltage. An oscilloscope can measure power or current indirectly, by using the known value of resistance across 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 kind of "measurement, " since it is qualitative as well as quantitative. If a wave shape "looks wrong," it might indicate distortion in a circuit, or possibly even betray a burned-out component someplace.