There aren't enough divisions on the scope screen to read the voltage. This is where the math trick comes in. We know the frequency of the sine wave and we can measure the slope of the wave as it crosses zero. Using that information, we can calculate the peak value. The equation for a sine wave is:
The derivative, or slope, of a sine wave is:
Now, if we know the frequency and measure the slope as rise over run on the oscilloscope screen, we can calculate the unknown peak value A.
Picture of the Day: