CALIBRATING AND MEASURING CIRCUIT FOR A CAPACITIVE PROBE-TYPE INSTRUMENT

A calibrating and measuring circuit is disclosed for a capacitive probe-type instrument. The capacitive probe, in contact with a surface to be measured, is connected to an oscillator which generates pulses, the frequency of which is adjustable, the number of pulses generated being a function of the texture of the surface being measured. A digital counter is connected to receive, count and present the resulting summation to a visual numerical display. In order to calibrate the instrument the capacitive probe is placed on a known specimen, the digital magnitude of which is loaded into the counter through a number of thumb switches. The counter first counts down to this number, and then counts up to the number of remaining pulses. The resulting count, called the offset number, is then transferred to a memory. During measurement the memory loads the offset number into the counter, the counter then counting the offset number down to zero, before counting up to the amount of the received pulses, which are then displayed. A logic arrangement initiates and controls both calibration and measurement.