GB690892A - Apparatus for measuring ground resistivity - Google Patents

Abstract

690,892. Resistance measurements; geophysical prospecting. SPINK, K. Feb. 12, 1952 [Feb. 12,. 1951], No. 3367/51. Class 37. Apparatus for measuring ground resistivity, particularly for geophysical prospecting, comprises a circuit adapted to send a current through the ground from a transformer energized by interrupted D.C. obtained from a battery, and a circuit adapted to measure the resulting potential difference between ground electrodes. The circuit for supplying current to the ground comprises a transformer 1, Fig. 1, to which interrupted D.C. is supplied from a battery 2 through a vibratory interruptor 3. The battery 2, which may be a 6-volt accumulator, is connected between a central tapping of the primary winding and a movable contact 4 of the vibrator 3 which engages alternately two fixed contacts 5 connected to the ends of the primary winding. A push-button control switch 6 is included in the battery circuit, and a pilot lamp 7 indicates when the current is on. The secondary winding of the transformer 1 has a number of tappings connected to a multi-contact switch 8 so that different output voltages may be applied to ground electrodes connected to terminals 9, 10. The current is measured by a moving-coil milliammeter 11 connected in a bridge-rectifier circuit comprising four selenium rectifiers 12. A resistance 13 may be connected across the output terminals 9, 10 by a switch 14 to provide an artificial load for testing the apparatus. Difference of potential due to the current supplied to the ground is picked up by ground electrodes connected to terminals 15, 16, Fig. 2 and is measured by a multi-range A.C. millivoltmeter formed by a moving-coil microammeter 17 connected in a bridge-rectifier circuit 26 and by a number of resistances 19 of different values, any one of which may be connected in series with the rectifier bridge by a selector switch 20. To compensate for natural ground potentials, a dry battery 21 is connected across the micro-ammeter 17 through a variable resistance 23, which can be adjusted to zeroize the micro-ammeter 17 before taking a reading. A reversing switch 24 enables the polarity of the current applied to the microammeter 17 from the battery 21 to be reversed.