A Stable Apparatus for Analyzing the Area of Polygraphic Curves

Abstract

The problem of analyzing polygraph-recordings has long troubled the experimenter interested in studies involving a recorded, fluctuating curve. Many attempts have been made to find a stable, accurate device which would analyze the polygraph-record in terms of curve-area. The planimeter and crude estimation have resulted in area-approximations which are both tedious and time consuming, while electronic efforts, in general, have not resulted in equipment of great stability. For example, Birmingham devised a rectifying type of integrator which, due to inherent difficulties in rectification circuits, is not as stable as desired over long periods of time.1 Adjustment of the integrator is also awkward and must be repeated at frequent intervals. The present paper describes a device which has great stability, is relatively inexpensive, and simple in construction. It requires little attention during operation. Mode of operation. The essential feature of the device is a highly stable feedback-amplifier circuit of the type used in analog-computers such as the Boeing-Electronic-Analog-Computer (BEAC).2 The arrangement used by the writers permits one to record curve-area in either a positive direction, a negative direction, or over total area of the curve. This is done by using two separate integrators, one for positive deflections, the other for negative deflections. These are switched automatically into the circuit according to voltage-direction and sign, since the integrator will normally operate in either direction. As the input-voltage swings about a zero-point, it is so rectified that, as it tends to go negative from the reference-point, it activates a sensitive relay which closes the input-circuit of Integrator A of Fig. 1., while the other integrator remains idle. In the positive direction the reverse occurs, hence one integrator serves to record all positive deviations above the reference-line, while the other structurally identical integrator records negative deviations of voltage.