Here is the trace for mains alternating current. One time period is worth 4 squares horizontally, since the oscilloscope plots a graph of voltage on the y-axis against time on the x-axis. The frequency of mains a.c. is 50 Hz. T=1/f so time period = 0.02 seconds or 20 ms. So one square is worth 5 ms. Inspection of the photograph reveals that the time-base setting is indeed 5 ms per division. The oscilloscope must have had the calibration knob in the correct place as well.
