Patent GB-841463-A (GB)
Title: Magnetometer apparatus
IPC: G01R33/24
Filed: 1958-11-04
Abstract: 841,463. Magnetometers; frequency measurement. VARIAN ASSOCIATES. Nov. 4, 1958 [Jan. 10, 1958], No. 35451/58. Class 37. In a free precession magnetometer of the kind in which atom portions are first polarized by a magnet field and then allowed to precess in the magnet field to be measured and thereby induce a field dependent signal in a receiver system, means are provided for automatically tuning the receiver system to the precession frequency. Fig. 1 shows an arrangement for measuring the earth's magnetic field. A sample of material 11, such as water, is surrounded by a coil 12 which is first connected to a power supply by means of a relay 14 so that the protons in the sample are aligned in a given direction, and then disconnected by the relay so that the protons precess at a frequency dependent upon the earth magnetic field. This frequency is measured by the apparatus and recorded on a recorder 22. The measurement is effected by passing the precession signal induced in the coil 12 through an amplifier 16 and a limiter 17 to a binary counter 18. When the counter starts to operate it simultaneously completes the circuit between a crystal controlled oscillator 19 and a second binary counter 21 and this second counter continues to operate until the precession signal counter has counted a predetermined number of cycles, whereupon a gate pulse generator 13 causes the output from the crystal oscillator to the second binary counter 21 to be short circuited so that the reading of the counter 21 represents the time taken by the predetermined number of precession signal cycles. The second binary counter 21 operates a fine tuner 24 in accordance with the count and so tunes the pre-amplifier to the precession signal for the period of the next measurement. In the detailed circuit (Fig. 2) the precession signal is applied through transformer 34 to a two stage amplifier 16, the output of which is applied through a first limiter 55 and a second limiter 17 to the precession signal counter 18. This counter ignores the first few cycles of the precession signal. and when it commences counting it applies a voltage through lead 58 and a cathode follower 25 to a diode 64 which is rendered non-conducting and so permits the output of a 100 kc/s crystal oscillator 19 to pass to a binary counter comprising a number of stages such as 65. When the precession signal counter 18 has counted 512 cycles it returns the voltage on lead 58 to normal so rendering the diode 64 conducting and short circuiting the path from the crystal oscillator to the binary counter which therefore ceases to count. The input and output to the amplifier 16 is coarsely tuned by a switch circuit 37, 38 while the output circuit is automatically finely tuned. As each counter stage 65 comes into operation an additional one of valves 78 is rendered conducting operating a relay 79 in its anode circuit, thus switching out inductance and capacitance 84, 86 from the output tune circuit 87, 88. The last tuning components 841, 86<SP>1</SP> are arranged to be either switched in or out of circuit by the relay 791 in accordance with the setting of the manual tuning switch to account for the overlapping of the ranges and this change in connection is effected by a relay 93 controlled by the manual switch. When the count has been completed and the signal at line 58 returns to normal the cathode follower feeds a voltage to a circuit 26 which operates a relay 28. One contact of this relay operates the polarizing relay 14 of Fig. 1, while the other contact restores the binary counting circuits to zero, leaving the tuning relays 79 operated in accordance with the previous count, ready for the next measurement.