THE C.R.T OSCILLOSCOPE
The cathode ray oscilliscope allows us to measure voltage over time, for instance a Oscilloscope set to a constant time base, connnected to an a A.C. circuit might show a trace like on the left. The peak voltage is half the vertical distance from the bottom of a wave crest to the top. The frequency is the time (proportional to distance) from one place on the wave to another one in phase with it. A double beam CRO, can be used to show two voltages at once, here the phase difference between them can be seen by measuring the distance between points in phase on the two waves.