constant current method

What Is the Constant Current Method?

The constant current method is an exosomatic technique for measuring electrodermal activity in which a constant, small electrical current is applied to the skin and the resulting voltage is measured. Because the current remains fixed, changes in voltage directly reflect changes in skin resistance (by Ohm's law: V = I × R). As eccrine sweat gland activity increases, skin resistance decreases, producing a measurable voltage change.

Comparison with Constant Voltage Method

The constant current method is the counterpart to the constant voltage method, which applies a constant voltage and measures changes in current flow (reflecting skin conductance). Both methods measure the same underlying phenomenon — changes in the electrical properties of the skin caused by sweat gland activity — but they express the measurement in different units (resistance in ohms versus conductance in siemens). Modern polygraph instruments typically use the constant voltage method to measure skin conductance, as conductance has a more linear relationship with sweat gland activity than resistance.

Technical Implementation

The constant current method uses electrodes placed on the fingers or palmar surface through which a very small current (typically microamperes) passes. The current level must be low enough to avoid skin polarisation effects or discomfort while being sufficient to produce measurable voltage changes. A Wheatstone bridge circuit is often used to detect and amplify the small voltage changes that occur with changes in skin resistance.

Historical Context

Both constant current and constant voltage methods have been used throughout the history of electrodermal measurement in polygraph testing. The choice between them was historically influenced by available electronics technology and instrument design. Modern digital polygraph systems predominantly use the constant voltage (conductance) method, which has become the standard in psychophysiological research and polygraph practice.