Patent GB-681214-A (GB)
Title: Improvements in or relating to apparatus for measuring small currents or voltages
IPC: G01R19/18
Filed: 1950-04-05
Abstract: 681,214. Current and voltage measurements; electric condensers. COLE, Ltd., E. K., and FINCH, H. W. S. April 5, 1951 [April 5, 1950], No. 8515/50. Class 37. In a dynamic electrometer of the type in which a capacitor, whose capacitance is made to vary cyclically by a fluctuating driving E.M.F., is employed to convert an applied D.C. potential to an A.C. potential, the natural frequency of vibration of the movable element of the capacitor is used as the frequencydetermining element of the system, so as to ensure that the frequency of the driving E.M.F. always bears a constant relationship to the natural frequency of vibration. A capacitor 1, Fig. 1, having a movable element, such as a vibrating reed of magnetic material, is connected across leads a to which the unidirectional voltage or current to be measured is applied. The circuit comprises a resistance R<SP>1</SP> and a highly insulated condenser C<SP>1</SP>, the fluctuations across which are impressed on an A.C. amplifier 2, the output of which is applied to a phasesensitive detector 3. The movable element of the capacitor 1 is energized by the coils L in a thermionic valve circuit 4. This serves as a standard-wave generator whose frequency is determined by the natural frequency of vibration of the movable element of the capacitor 1. A connection is taken from the generator 4 to the detector 3 for comparison with the output from the amplifier 2. The output from the detector 3 is fed to a cathode follower 5 whose output is measured by a voltmeter V. A negative feed-back circuit 6 ensures that variations in the gain of the amplifier &c. have minimum effect on the readings, so that the instrument is suitable as a null detector. A suitable construction of vibrating-reed capacitor is shown in Fig. 2. One plate M of the capacitor is mounted on a polystyrene block U and is fitted with a screwed shank W to permit adjustment of the gap. The movable element is a stainless steel reed O<SP>1</SP> vibrated by an electromagnet P which is energized by the coils L, Fig. 1, and is provided with a screen S. One end of the reed O<SP>1</SP> is fixed to the casing at T. The condenser C<SP>1</SP>, Fig. 1, is made integral with the assembly of the fixed plate M and is of moulded construction, having a polythene dielectric and possessing high insulation resistance. In the construction shown in Fig. 3, a circular permanent magnet, N, with concentric polepieces, carries one of the coils L wound on an insulating former fixed around the central polepiece. The other coil is wound on a cylindrical former Q suspended at T by an internal diaphragm and constrained so that the coil is free to move only in the central plane of the central polepiece. The former Q also carries a metal plate O<SP>2</SP> which forms the movable plate of the capacitor. The fixed plate M is fitted with a screwed shank W and is carried by an insulating support U which is rigidly connected to the magnet N by means not shown. The coils L are connected in a thermionic valve circuit as shown in Fig. 5. An alternating voltage is generated in the coils so that the suspended coil is alternately attracted to and repelled from the fixed coil and imparts oscillatory motion to the plate O<SP>2</SP>. In the construction shown in Fig. 4, a non-magnetic casing R encloses a permanent magnet V on which are mounted the coils L. Over the top of the polepieces is a tightly stretched diaphragm Y of magnetic material which may form the movable plate of the capacitor or, as shown, may be mechanically coupled to a separate plate O<SP>3</SP>. Specification 550,610 is referred to.