Patent GB-236921-A (GB)
Title: Improvements in frequency standards
IPC: G01R23/02
Filed: 1925-06-29
Abstract: 236,921. Western Electric Co., Ltd., (Assignees of Ricker, N. H.). July 9, 1924, [Convention date]. Tuning.-A frequency standard comprises an elastic bar mounted for free-free vibrations, i.e. supported at nodal points so that the ends are free to vibrate. In the construction shown in Fig. 1, a steel bar is supported at nodal points by knife-edges 3 of a support 2 and is set into transverse oscillation by an electromagnet 4 excited from a source of alternating current 7. The acoustic waves set up by the bar 1 are converted into corresponding electric currents by a telephone receiver 6. These currents, which may be amplified by an amplifier 11, are passed through one, of a pair of impedances 9, the other of which is in series with the source of alternating current 7. The telephone receiver 6 is adjusted so as to be spaced from the rod 1 by a quarter of a wave length of sound in air. The potentials across the impedances 9 are then initially displaced by 90‹ and are adjusted to equality. As the frequency of the source 7 is increased to produce maximum resonance of the bar 1, a phase shift of 180‹ occurs at the instant when coincidence occurs between the frequency of the source 7 and the resonance frequency of the bar 1 and is indicated by a momentary cessation of sound in a telephone receiver 12 connected across the impedances 9. Alternatively the vibrations of the bar 1 may generate currents in coils wound on a permanent magnet, which currents may be amplified and rectified and indicated on a direct-current meter, the maximum reading indicating maximum resonance. The knife-edges 3 may be replaced by trunnion supports. The amplitude of the vibrations of the bar 1 may be increased by additional electromagnets at the free ends acting on the upper surface of the bar. Maximum resonance of the bar 1 may be indicated by direct observation with a microscope ,or by observation of the maximum sound by a stethoscope. In a modification, Fig. 11, the bar 1 forms one element of a condenser the other element 23 of which is insulated from its support, which is adjustable by screw means. A direct-current source 24 is applied between the condenser elements 1, 23, resonance of the bar 1 being indicated by a maximum value of the oscillations set up in the circuit. The bar 1 may be mounted so as to oscillate longitudinally instead of transversely. Fig. 9 shows such a construction in which the bar 1 is supported at its central nodal point and is caused to vibrate by electrostatic attraction between the end of the bar and an insulated conducting part 23 connected to one terminal of the source 7. A similar condenser is formed by the other end of the bar 1 and a similar conducting part 23. This condenser is polarized by a direct-current source 24, oscillatory currents being caused by vibrations of the bar. Longitudinal oscillations in a steel bar may be excited by an electromagnet acting on the end surface. The bar may be mounted so as to create nodal points one quarter of the length of the bar from the ends. The bar 1 may be formed of a transparent isotropic elastic material such as glass, in which case the maximum resonance mav be indicated by observing the effect on a beam of polarized light by the strain produced in the glass. The glass bar may be caused to vibrate longitudinally at its higher harmonic rates, the frequency being determined by counting the number of nodes. With nodes 5 mm. apart, the bar vibrates at a frequency greater than 100 kilocycles. A bar cut from a uniaxial or biaxial transparent crystal may be employed as a vibrating body, and may be set into vibration piezoelectrically.