to buy two E-meters, in the event that one of them fails to operate. According to anthropologist Roy Rappaport, the E-meter is a ritual object, an object that "stand[s] indexically for something intangible". Functional description One of E-meter's primary components is a Wheatstone bridge, an electrical circuit configuration invented in 1833 that enables the detection of very small differences between two electrical impedances (in this case, resistances). The E-meter is constructed so that one resistance is the subject's body and the other is a rheostat controlled by the operator. A small voltage from the battery is applied to electrodes held in the subjects hands. As the electrical properties (electrodermal activity) of the subject's body changes during the counseling, the resulting changes in the small electric current are displayed in needle movements on a large analog panel meter. The dial face is without numbers because the absolute resistance in ohms is relatively unimportant, while the operator watches primarily for characteristic needle motions. The voltage applied to the electrodes is less than 1.5 V, and the electric current through the subject's body is less than a half a milliampere. In the Scientology E-meter, the large control, known as the "tone arm", adjusts the meter bias, while a smaller one controls the gain. The operator manipulates the tone arm to keep the needle near the center of the dial so its motion is easily observed. A simple E-meter powered by direct current, such as that used by the Scientologists and the like, displays several kinds of electrodermal activity (EDA) on the one dial without distinction, including changes in conductance, resistance, and bioelectric potential. Researchers in psychophysiology are also exploring admittance and impedance aspects of EDA that can be observed only with alternating current. The E-Meter, measuring variations in electrodermal activity (which can be highly responsive to emotion), functions on the same physiological data sources as one of the parts of the polygraph, or “lie detector”. According to Scientology doctrine, the resistance corresponds to the "mental mass and energy" of the subject's mind, which are claimed to change when the subject thinks of particular mental images (engrams). One account tells about Hubbard using the E-meter to determine whether or not fruits can experience pain, as in his 1968 assertion that tomatoes "scream when sliced". The traditional theory of EDA holds that skin resistance varies with the state of sweat glands in the skin. Sweating