V. METHOD OF STUDY.
The measurements of reaction time herein considered were made with the Hipp Chronoscope. Cattell's 'Falling Screen' or 'Gravity Chronoscope' was used as a control for the Hipp. The Gravity Chronoscope consists of a heavy metal plate which slides easily between two vertical posts, with electrical connections so arranged that the plate, when released from the magnet at the top of the apparatus, in its fall, at a certain point breaks an electric circuit and at another point further down makes the same circuit. The rate of fall of the plate is so nearly constant that this instrument furnishes an accurate standard time with which Hipp readings may be compared, and in accordance with which the Hipp may be regulated. For, since the rate of a chronoscope varies with the strength of the current in use, with the variations in temperature and with the positions of the springs on the magnetic bar, it is always necessary to have some standard for corrections. In these experiments the time of fall of the gravity chronoscope plate, as determined by the graphic method with a 500 S.V. electric tuning fork, was 125σ (i.e., thousandths of a second).
This period, 125σ, was taken as a standard, and each hour, before the beginning of reaction-time experiments, the time of the plate's fall was measured ten times with the Hipp, and for any variation of the average thus obtained from 125σ, the standard, the necessary corrections were made by changing the position of the chronoscope springs or the strength of the current.
The standard of comparison, 125σ, is shorter than most of the reaction times recorded, but since the time measured was always that from the breaking to the making of the circuit passing through the chronoscope it cannot be urged that there were errors resulting from the difference of magnetization which was caused by variations in the reaction time. But it is evident that the danger from differences in magnetization, if such exists, is not avoided in this way; instead, it is transferred from the reaction time proper to the period of preparation immediately preceding the reaction; for, from the moment the chronoscope is started until the stimulus is given a current is necessarily passing through the instrument. At a verbal signal from the operator the assistant started the chronoscope; the stimulus was then given by the operator, and the instrument recorded the time from the breaking of the circuit, effected by the stimulating apparatus, to the making of the circuit by the reaction of the animal. Despite precautions to prevent it, the period from the starting of the chronoscope to the giving of the stimulus was variable, and errors were anticipated, but a number of the tests proved that variations of even a second did not cause any considerable error.
A fairly constant current for the chronoscope was supplied by a six-cell 'gravity battery' in connection with two storage cells, GB (Fig. 6). This current could be used for two hours at a time without any objectionable diminution in its strength. The introduction of resistance by means of the rheostat, R, was frequently a convenient method of correcting the chronoscope.
Fig. 6. General Plan of Apparatus in Diagram. H, Hipp Chronoscope; R, rheostat; C, commutator; SC, storage cells; GB, 'Excello' gravity battery; F, Cattell's falling screen; T, reaction table; RK, reaction key; SK, Stimulating apparatus; K, key in chronoscope circuit; S, stimulus circuit.
Fig. 6 represents the general plan of the apparatus used in these experiments.
The general method of experimentation is in outline as follows:
1. At a 'ready' signal from the operator the assistant makes the chronoscope circuit by closing a key, K (Fig. 6), and then immediately starts the chronoscope.