1,073,767. Electrophysical measurement, control &c. LABORATORY EQUIPMENT CORPORATION. May 13, 1966 [May 14, 1965], No. 21475/66. Heading G1N. The invention relates to the determination of e.g. urea or glucose in a blood sample, by measuring the variation in conductivity at a predetermined time during an enzyme reaction. The conductivity cell 38, (comprising electrodes lowered into a beaker, ) is connected in a differential circuit with a balancing resistor 78 across the secondary of a transformer 68. The output is fed through an amplifier and rectifier to a meter M. An amplifier gain control 88 is ganged to the balancing resistor, and a "calibrating" resistor 84 is provided. Supply circuits for a stirrer motor 114, timer 14, and meter clamping device 140 are shown. Calibration:- urease is added to a standard serum containing a known urea nitrogen content, and the timer is started. For the first 20 seconds the ganged balancing/gain control is turned to keep the meter at zero. Then for the next 60 seconds the "calibration" gain control is used to maintain the meter deflection at the known percentage content of the sample until the meter clamp is operated by the timer. Measurement of urea content is made after calibration by mixing urease with the sample, starting the timer and maintaining the meter at zero for 20 seconds by the balancing/gain control as before. The meter is then allowed to deflect until the clamp operates, when a direct reading of the urea content is obtained. Modification; for the measurement of glucose in a sample (which is decomposed by glucose oxidase to gluconic acid) the sample is first deionized by passing through an ion exchange resin "filter" to remove other normally present conductive ions which might lead to inaccurate results.