Patent GB-771958-A (GB)
Title: Improvements in and relating to apparatus for measuring electrical quantities which determine magnetic field intensities
IPC: G01R33/24
Filed: 1953-12-07
Abstract: 771,958. Electromagnets. GENERAL ELECTRIC CO. Dec. 7, 1953 [Dec. 6, 1952], No. 33996/53. Class 35. [Also in Group XXXVI] Apparatus for measuring an electrical quantity proportionate to variations of magnetic field intensity comprises magnetic circuits for producing substantially constant and similar magnetic fields at two discrete points to intersect specimens of a substance whose constituent protons or electrons exhibit Larmor magnetic resonance or precession about the field axis due to interaction therewith of the angular momenta and the magnetic moments of the nuclei of the specimens, which are surrounded by coils, orthogonally of the main field, energized at the Larmor magnetic resonance frequency by a radio frequency oscillator through corresponding T-bridge circuits such that unbalance signals are produced when a bridge is unbalanced due to the absorption of energy and consequent increase in apparent resistance of the appropriate specimen coil at the Larmor resonance frequency; the intensity of the main field in each specimen being regulated by a pair of 1st field windings substantially to the value for magnetic resonance at the excitation frequency, a 2nd winding energized proportionately to the electrical quantity to be measured controlling the field in one specimen only, and a pair of 3rd windings amplitude modulating the field in each specimen at an audio-frequency whereby the output signals of the bridges are similarly modulated and are respectively rectified and utilized to control the excitation of the pair of 1st windings to maintain the average field intensity in both specimens at the correct value for Larmor resonance, and to simultaneously control a secondary means for producing a magnetic field neutralizing the variations of the field in the second specimen which are derived from the winding energized by the electrical quantity to be measured, so as to restore the field intensity to the Larmor resonance value; the controlling signal being measured as a function of the quantity to be measured. Fig. 6 shows a magnetic circuit for use in the apparatus wherein a cylindrical ferromagnetic housing 152 having ferromagnetic end plates 153, 154 contains cylindrical permanent magnets 155, 156 with soft iron polepieces 158, 159 separated by an air gap containing a transverse plastic non-magnetic spacer 164 with an axial cavity into which fits a non-magnetic plastic rod 160 into the head of which is sealed a specimen 157 of a substance (e.g. a 1/250 molar aqueous solution of manganous sulphate) exhibiting Larmor magnetic resonance at a frequency at which coil 162 surrounding rod 160 is energized from an R.F. oscillator over coaxial cable 163, secured in position by screw 168, while the opposite opening in housing 152 is closed by plug 167 secured by screw 169. The field windings 172, 173 surround the magnets 155, 156 while the modulating windings 170, 171 are located within annular grooves in the faces of spacer 164. Screws 165, 166 apply pressure for gap adjustment. Figs. 7, 8 show a modified magnetic circuit in which tapered soft iron polepieces 174, 175, 176 are axially aligned with two gaps for specimens 196, 197 surrounded by Larmor frequency coils 198, 199, between ferromagnetic end plates 193, 194 of a ferromagnetic cylinder 195. Bar magnets 177 to 184 extend radially of the central N polepiece 175 to vertical soft iron strips 185 to 192 attached to the inside of the outer cylinder to close the magnetic circuit. Field coils 204, 205 surround polepieces 174, 175 while the modulating windings comprise annular coils 200 to 203 positioned at the tips of the several polepieces. The gaps are adjustable by applying pressure to the end plates.