#Electric recorder.#--We have thus a method of obtaining curves of response electrically. After all, it is not essentially very different from the mechanical method. In this case we use a magnetic lever (fig. 4, _a_), the needle of the galvanometer, which is deflected by the electromagnetic pull of the current, generated under the action of stimulus, just as the mechanical lever was deflected by the mechanical pull of the muscle contracting under stimulus. The accompanying diagram (fig. 4, _b_) shows how, under the action of stimulus, the current of rest undergoes a transitory diminution, and how on the cessation of stimulus there is gradual recovery of the tissue, as exhibited in the return of the galvanometer needle to its original position. [Illustration: FIG. 4.--ELECTRIC RECORDER (_a_) M muscle; A uninjured, B injured ends. E E' non-polarising electrodes connecting A and B with galvanometer G. Stimulus produces 'negative variation' of current of rest. Index connected with galvanometer needle records curve on travelling paper (in practice, moving galvanometer spot of light traces curve on photographic plate). Rising part of curve shows effect of stimulus; descending part, recovery. (_b_) O is the zero position of the galvanometer; injury produces a deflection A B; stimulus diminishes this deflection to C; C D is the recovery.] #Two types of response--positive and negative.#--It may here be added that though stimulus in general produces a diminution of current of rest, or a negative variation (e.g. muscles and nerves), yet, in certain cases, there is an increase, or positive variation. This is seen in the response of the retina to light. Again, a tissue which normally gives a negative variation may undergo molecular changes, after which it gives a positive variation. Thus Dr. Waller finds that whereas fresh nerve always gives negative variation, stale nerve sometimes gives positive; and that retina, which when fresh gives positive, when stale, exhibits negative variation. The following is a tabular statement of the two types of response: I. _Negative variation._--Action current from more excited to less excited--cuproid change in the excited--e.g. fresh muscle and nerve, stale retina. II. _Positive variation._--Action current from less excited to more excited--zincoid change in the excited--e.g. stale nerve, fresh retina.[4] From this it will be seen that it is the fact of the electrical response of living substances to stimulus that is of essential importance, the sign _plus_ or _minus_ being a minor consideration. #Universal applicability of the electrical mode of response.#--This mode of obtaining electrical response is applicable to all living tissues, and in cases like that of muscle, where mechanical response is also available, it is found that the electrical and mechanical records are practically identical.