1. An instrument that measures and records brain wave or the electrical potentials on the scalp and generates a record of the electrical activity of the brain.
2. An apparatus for receiving and recording the electric potential produced by the brain cells.

It consists of a vacuum tube amplifier that magnifies the electrical currents received through electrodes placed on the scalp and electromagnetically records the patterns on a graphic chart.

3. An appliance that is used to amplify and to record the electrical activity of the brain, either through the intact skull and scalp, or directly from the exposed cerebral cortex (electrocorticography).

It is made up of a number of channels, each of which consists of an amplifying and recording instrument that records the electrical events beneath and between a pair of recording electrodes.