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PROCEEDINGS OF THE PHYSIOLOGICAL SOCIETY

Platinum electrodes, inserted into various tissues of an anaesthetized rat, take up definite potentials (Cater & Phillips, 1954) which show characteristic changes following the administration of oxygen and of certain drugs. An apparatus has been developed to record the potentials at eight electrodes. The electrodes for oxidation-reduction potential measurements are of platinum or other noble metal, and a small glass electrode with a pointed tip is used to record pH changes. Measurements are made relative to a subcutaneous silversilver chloride electrode which is earthed. The electrodes rapidly become polarized by currents as small as 10-9 amp, and good insulation is therefore essential. Either glass, or 'Araldite 985 E', is used to insulate the electrodes, and polythene-insulated coaxial cable is used for leads. Motor-driven switchgear connects four electrodes in turn to each of two separate electrometer valve circuits with input impedance 1012 ohms. These circuits operate the two moving coils of a 'Cambridge' 4-channel recording galvanometer, which records every two minutes on each of 8 electrodes, or proportionately more frequently if fewer electrodes are in use. The sensitivity normally used is 2 mV per millimetre and drift of the zero is less than 1 mV/hr. The range of measurement can be altered at will, independently for each channel, by applying known voltages to the grid of the idling valve (see Fig. 1). An extension of the apparatus allows a known small voltage to be applied to any one electrode, and the current to be recorded by the galvanometer, which