Patent GB-472116-A (GB)
Title: Improvements relating to the determination of electrical values
IPC: G01R1/28
Filed: 1936-03-11
Abstract: 472,116. Photo-electric apparatus for measuring electrical values. WULFF, P., and KORDATZKI, W. March 11, 1936, No. 7355. [Class 40 (iii)] [Also in Group XXXVI] Electric potential, resistance, or current is measured by adjusting a potential or ratio of potentials to a value such that the current flowing in a circuit is zero or very small, the potential or potentials being obtained from one or more photo-electric cells of the current generating type, without any battery or similar source of voltage supply, and the illumination conditions being varied automatically in accordance with the current conditions in the circuit, so as to adjust the potential or potentials automatically to the desired value whereupon this value is determined by voltage measuring means associated with the cell or cells. In one form, the unknown potential difference and the photo-electric cells are connected in opposition and in series with a moving coil galvanometer, the pointer of which carries a slotted screen adapted to move between two slotted diaphragms, one of which is situated at the end of the illuminating tube and the other at the end of the tube carrying the photoelectric cell. The galvanometer has no tension or like control and when a current flows in the coil, the pointer moves in such a direction as to increase or reduce the output of the photoelectric cell until the current is reduced to zero. In this position of the pointer and the screen carried thereby, the output of the cell is equal to the potential difference to be determined and is measured by a voltmeter across its terminals. The potential difference to be found may be the pH value of a liquid, the voltmeter if desired being graduated directly in such values and a standard cell may be suitably connected in the circuit if the unknown potential difference should be too great or too small to be compared with the voltage of the photo-electric cell. In a modification for measuring resistances, two cells P1, P2 may form two arms of a Wheatstone bridge circuit, the other arms of which are formed by a known resistance R2 and an unknown resistance R1. A moving coil galvanometer 11 is connected in one diagonal of the bridge and light from a source G is reflected from the mirror of the instrument on to the cells P1, P2. When a current flows through the galvanometer, its coil is rotated and more light falls on one of the cells, tending to reduce the current to zero. When no current flows in the coil, the ratio of the resistances is equal to the ratio of the voltages provided by the cells and this is determined by the cross-coil voltmeter 18 which may be graduated to provide a direct reading of the unknown resistance. The galvanometer pointer may be provided with two screens, one for shielding each of the cells P1, P2. In a modification, one of the cells P1 is replaced by a standard battery cell and the galvanometer pointer carries only one screen which serves to vary the voltage of the photoelectric cell P2 until no current flows through the galvanometer, the voltage of the cell being then measured by means of a voltmeter. Specification 399,641, [Group XXXVI], is referred to.