1) Any manipulable object, the movement of which closes or breaks an electrical circuit. In experiments with pigeons, a useful key is a translucent disc at a convenient height on the wall of the experimental box. When the pigeon pecks this disk, a circuit is made or broken. In experiments with rats, a horizontal bar parallel to and approximately inch from the wall of the experimental box can be pressed downward against a light spring to close or break the circuit.

2) A hinged plate which produces an electrical pulse when moved is called a key. In experiments with pigeons, a translucent disc at a convenient height on the wall of the experimental box is frequently used as a key. When a pigeon pecks this disc, the movement operates an electrical switch. A spring returns the hinged plate to the unoperated position. In experiments with rats, a horizontal bar, parallel to the wall of the experimental space, closes the switch against the pressure of a light spring.