Apparatus for the measurement of the GSR may be divided into two main groups, depending on whether alternating current (Ac) or direct current (DC) is applied to S. AC offers two major advantages over DC. (1) It eliminates polarization, permitting the use of dry silver electrodes; and (2) it makes practical the use of electronic amplifiers which increases the sensitivity beyond that obtained by the usual DC-bridge. Amplifiers also reduce the size of the current that must be applied to S, thereby reducing the possibility of disturbing the effect by the act of measuring it. In preliminary experiments it was found that both the resistance-level of the skin and the relative GSR decrease with increasing frequency. The decrease of the resistance-level can be roughly approximated by a negative exponential equation. The GSR also decreases with frequency hence there is only a small indication left at 1000 ~. Although the indication of the GSR is less with AC than with DC, the response at 60 is quite satisfactory. The basic resistance of the skin at this frequency was found to be about one-half of that obtained with DC, and it may be assumed that a corresponding decrease in the size of the GSR also occurs. In spite of the smaller indications obtained, it was felt that the other advantages of AC justified its use. Since the GSR consists of a relatively small change in a large static level of skin resistance, it is necessary to cancel a large portion of the initial resistance-level to obtain an accurate measure of the change that is of interest. This is done easily in De-apparatus by means of a bridge. ACbridges, on the other hand, are both elaborate and expensive, since it is necessary to balance both the resistive and reactive components of the impedance. This balancing is quite time-cosuming, and since GSR's last only a matter of seconds, the usual Ac-bridge is impractical. The AC-circuit devised by Grant offers an ingenious solution to the balancing problem.' Grant used AC across S, but rectified this potential,