Patent GB-941310-A (GB)
Title: Methods of measuring weak magnetic fields by nuclear induction
Inventor: BONNET GEORGES
IPC: G01R33/24
Filed: 1959-12-17
Abstract: 941,310. Magnetic tests. COMMISSARIAT A L'ENERGIE ATOMIQUE. Dec. 17, 1959 [Dec. 23, 1958], No. 42909/59. Heading G1N. Apparatus for measuring weak magnetic fields by measuring the frequency of nuclear precession of the protons of a sample of liquid containing dissolved in it a paramagnetic impurity having a line of electronic resonance having hyperfine structure, comprises first and second windings, the axes of which are mutually perpendicular and perpendicular to the direction of the field to be measured, means for applying a high-frequency alternating voltage to the first winding to produce a magnetic field to saturate said electronic resonance line, means for applying a direct voltage to the second winding to produce a weak polarising magnetic field, means for rapidly suppressing the polarising magnetic field in a short time compared with the duration of nuclear precession of the protons, when the direct voltage is cut off, and means for connecting the second winding to a frequency measuring instrument, the signal received by the latter instrument being the Larmor frequency of nuclear precession of the protons. A high frequency coil 5 fed from oscillator 1 is situated inside flask 2 containing the paramagnetic sample so as to saturate one of the lines of a dissolved paramagnetic salt such as potassium nitrosodisulphonate, the coil's axis being perpendicular to the field H to be measured i. e. 7. Coil 8 wound round the flask 2 and having its axis perpendicular to both the coil 5 and the field H serves to produce an auxiliary magnetic field which alters the orientation of the nuclear spins of the sample causing them subsequently to precess and also to pick up the signal which is induced when precession occurs. This signal is transferred via relay box 9 to amplifier 10 tuned approximately to the resonant frequency of the protons in the field H and then to frequency meter 11. In operation, direct current is fed to the low frequency coil 8 so as to orientate the resultant magnetisation of the protons perpendicular to the field H,the amplifier 10 being short-circuited and the counter 11 set to zero. The auxiliary field is subsequently cut off so as to enable the resultant magnetisation to precess round the field H, the coil 8 at the same time being switched over to the terminals of the amplifier 10 and frequency measurer for measurement of the induced signal. Switching mechanism. The switching circuit for cutting off the auxiliary field is as shown in Fig.3. At the start of the operation the double triode 19 is driven into conduction by positive bias applied to its grid over contact 17, and relay 16 energizes relay 21 to connect coil 8 to the battery via valve 19 and at the same time connect the input of amplifier 10 to earth. When relay 16 is de-energized valve 19 is cut off and coil 8 de-energized. The amplifier remains short-circuited during the time in which the resultant magnetisation is reorientating due to capacitor 30 delaying the drop out of relay 21. When relay 21 does finally drop out coil 8 is connected to amplifier 10 via connector 31 enabling the precession frequency to be measured. The valve 19 may be replaced by a transistor. Reference has been directed by the Comptroller to Specification 888, 439.