Method and apparatus for measuring the amount of weak magnetic fields, in particular of the earth field, by means of core induction

摘要

964,017. Nuclear magnetic resonance apparatus. COMMISSARIAT A L'ENERGIE ATOMIQUE. July 3, 1961 [July 5, 1960: July 18, 1960], No. 23974/61. Heading G1N. In a free precession magnetometer using a solution containing atomic nuclei having a definite gyromagnetic ratio different from zero and paramagnetic ions with an unpaired electron having a saturable electronic resonance line at a frequency different from zero when in a magnetic field tending to zero in which an alternating field is applied to saturate the electronic resonance line of the latter and the unidirectional polarising field is of the same order of intensity as the field to be measured (one to ten times) a switching arrangement is provided to connect a D.C. source to the polarising field coil to quickly transfer the coil to an energy dissipating circuit and then to a frequency measuring circuit, the switching arrangement being controlled so that the period of connection of the source to the coil is of the same order as or of the period of the Larmor frequency. The saturating field for the paramagnetic ions which is in the region of 55. 8 megacycles is supplied to a coil 2 Fig. 1, comprising 4 turns of heavy gauge wire by a generator 5 through a coaxial cable 4 and a matching network 3, and the polarising field is applied to a coil 6 from a D.C. source 9 through the contacts 10 of a relay controlled by clock pulses from a generator 17. A further set of relay contacts 11 also controlled by generator 17 connects a resistor 12 across coil 6 when contacts 10 are opened to dissipate the stored energy, and then short circuits the resistor to enable the Larmor frequency to be applied through a selective amplifier 7 to a frequency and recording apparatus 15, 16. Capacitors 13, 14 tune the coaxial cable 8 to the Larmor frequency, and selective and adjustable band-pass stages are included in the amplifier to cater for different ranges of field strength to be measured. The control of relays 10 and 11 by pulses from generator 17 is achieved by a pair of double triode 20, 31 Fig. 4 to which the control pulses are commonly applied and which include the relay coils 10, 11 in series with protective or isolating condensers 25, 35 in their cathode circuits, or pulse elongating network 29, 30 being included in the grid circuit of valve 31. Terminals 18, 27 and 37 connect to apparatus 17, 6 and 7 of Fig. 1 respectively, the D.C. power being supplied from source 9 through double triode 26 the grid bias of which is adjustable by the wiper of a potentiometer 39. An alternative arrangement using transistor switches is described with reference to Fig. 5 ( not shown) and lists of suitable paramagnetic substances and component valves are included. Electric waveforms are described with reference to Figs. 2 and 3 (not shown). Speci - fications 888, 439 and 941,310 are referred to.