reaction-key

from The Century Dictionary.

• noun In physiology and psychophysics, an instrument by means of which the movement of response in the reaction experiment is executed and the moment of this reaction marked upon the recording apparatus (chronograph or chronoscope). The reaction-keys in common use are so constructed that the movement of reaction (pressure or lift of a finger) makes or breaks an electric circuit. Other keys are arranged for air-transmission, or for direct mechanical transmission by means of levers. The electrical keys are also made for a large variety of responsive movements, as lip-keys, speech-keys, voice-keys, eyelid-keys, etc.