What is Amplitude?

Example 8.5 (Mains and concert pitch)

The mains voltage oscillates at $f = 50\,\mathrm{Hz}$: its period is $T = 1/f = 0.020\,\mathrm{s} = 20\,\mathrm{ms}$. An orchestra’s concert A is a pressure signal at $440\,\mathrm{Hz}$: one cycle lasts $T = 1/440 = 2.3\,\mathrm{ms}$. Higher frequency, shorter cycle: $f$ and $T$ are inverses of each other.

Example 8.8 (A full reading)

On the oscillogram below, the time base is $5\,\mathrm{ms}$ per division and the gain $2\,\mathrm{V}$ per division. One cycle spans $4.0$ divisions: $T = 4.0 \times 5\,\mathrm{ms} = 20\,\mathrm{ms}$ and $f = 1/0.020\,\mathrm{s} = 50\,\mathrm{Hz}$ — the mains again. A crest sits $3.0$ divisions above the axis: the amplitude is $3.0 \times 2\,\mathrm{V} = 6.0\,\mathrm{V}$.