The pressing of the button by him breaks the current and stops the clock.

The dial of the clock indicates the actual time which has elapsed between the bell (signal) and his response with his finger (reaction).

The clock used for exact work is likely to be the Hipp chronoscope, which gives on its dials indications of time intervals in thousandths of a second.

For the sake of keeping the conditions constant and preventing disturbance, the wires are made long, so that the clock and the experimenter may be in one room, while the bell, the punch key, and the subject are in another, with the door closed.

This method of getting reaction times has been in use for a number of years, especially by the astronomers who need to know, in making their observations, how much time is taken by the observer in recording a transit or other observation.