Which instrument displays electrical voltage as a waveform over time?

An oscilloscope displays electrical voltage as a waveform over time.

The instrument presents voltage on a vertical axis and time on a horizontal axis. This lets users inspect features such as amplitude, frequency, period, phase difference, rise time, and noise. Modern digital oscilloscopes sample an input signal, store the measurements, and render them on a screen.

Earlier oscilloscopes used cathode-ray tubes. An electron beam moved across a fluorescent screen while the input voltage deflected the beam vertically, creating a visible trace. The horizontal sweep repeated rapidly enough for periodic signals to appear stable.

An oscilloscope is different from an ordinary voltmeter. A voltmeter usually reports a numerical voltage, while an oscilloscope reveals how voltage changes. It can also compare two signals, helping diagnose circuits, audio equipment, communication systems, and digital electronics.