Oscilloscopes measure the shape of electric signals and display a moving graph called a trace that represents the voltage against time on a display. The varying electrical signal is picked up and sent to an electron gun. Here, a negative cathode ray is shot down a tube to the screen, accelerated by a positive anode, where the waveform is displayed. Waveforms can then be manipulated on both the horizontal and vertical axes to accommodate different inputs.
In the case of medical oscilloscopes, electrodes attached to the skin pick up the regular electric variations in a heartbeat on a special oscilloscope called an ECG (electrocardiograph). One healthy heart should have the same rhythm as the next, producing a similar waveform, so, by analysing the output, doctors can quickly get an indication of potential problems.