Liquid developer charge director control

Abstract From the Canadian patent

A copier wherein charge director is supplied to a liquid developer in response to a conductivity measurement thereof. The toner concentrate is deficient in charge director and is supplied to the liquid developer in response to an optical transmissivity measurement thereof. Conductivity is measured by a pair of spaced electrodes immersed in the developer and excited by a variable alternating voltage source through a fixed resistor. The effects of inherent capacitance between electrodes are neutralized by a variable capacitor. A phase sensitive detector is provided with a reference voltage having the same phase shift as occasioned by capacitive effects. The copier is disabled if developer …