GB612813A - Improvements in or relating to apparatus for electrical measurement - Google Patents

Abstract

612,813. Prospecting for ores; resistance measurements. TRIGGS, W. W. (Bolidens Gruvaktiebolag.). Dec. 4, 1944, No. 24215. [Class 37] For detecting subterranean deposits or for determining the resistance of electrolytes, ores and rocks, a current is employed which varies linearly with time. A Wheatstone bridge may be supplied with such a current. Fig. 3 shows apparatus for determining the potential distribution around a current electrode 7 put into the ground or into some other material. Linearly varying current is supplied from a source 1 to the ground through electrodes 6, 7 and a certain voltage is obtained between two potential electrodes 8, 9 put into the ground adjacent to the current electrodes 6, 7. This voltage is compensated by a voltage induced in a coil 10 and adjusted by a potentiometer 11 so that the instrument 2 reads zero. Figs. 4a, 4b and 4c show an arrangement for measuring electromagnetic fields. Linearly varying current from a source 1 flows through a coil 16, and the voltage induced in a frame 17, Fig. 4a, is compensated by the voltage from a coil 14 by means of a potentiometer 15. The relationship between the electromagnetic field at two points outside the coil 16 may be determined by comparing the voltages induced in two frames 18, 19, Fig. 4b. The direction of the field may be determined by inclining a frame 20, Fig. 4c. The currents employed-may be aperiodic when the measurement has to be performed in a very short time before the current and voltage reach high values. Alternatively, the currents may be periodic as shown in Fig. 5. A current 1 rising and falling rectilinearly with time is sent through a rectilinear cable, about which an electromagnetic field is produced. In a coil placed within this field a voltage 2 is induced. If there exists also an electromagnetic field from another conductor, a voltage 3 is induced, the resultant 4 of these voltages being measured by a compensator apparatus as described above. The voltage from the secondary field can be determined if the strength of the primary field is known. Currents of the required type may be produced by special generators or by means of rotating resistance discs. Alternatively, the substantially straight portion of the charge or discharge curve of a condenser may be employed, or a rhombic frame may be inserted with constant speed in a homogeneous magnetic field.