Patent GB-342441-A (GB)
Title: Improvements in alternating-current potentiometers
IPC: G01R1/28
Filed: 1929-12-02
Abstract: 342,441. Potentiometers; current meters; calebrating instruments; inductance coils ; adjusting wireless apparatus. CAMPBELL, A., Culmora, Girton Road, Cambridge. Dec. 2, 1929, Nos. 36837/29 and 13408/30. [Class 37.] An alternating current potentiometer of the polar type consists of a phase splitter connected to two primary coils A, B, Fig. 2, variably coupled to a secondary coil D which is divided into sections, by stranding or otherwise, connected to stud contacts of a switch for giving steps in the magnitude of the induced voltage as shown in Fig. 1. For fine adjustment one or more sections are connected to a potentiometric rheostat W. In order to enable the currents fed to the potentiometer to be varied from zero to maximum while keeping the load on the source constant, there is introduced between the source and the phase splitter a primary circuit with variable couplings to the potentiometer circuit and to a dummy circuit, the relation of the couplings being such that the sum of the squares of the secondary currents is kept constant. One form of phase splitter is shown in Fig. 6 in which the primary coil of a mutual inductance M and a resistance s are connected in series with the source. One of the primary coils A of the potentiometer is connected to the secondary coil of the mutual inductance, the other primary coil B being connected in parallel with the resistance s. The resistance s and the mutual inductance M are adjustable for various frequencies. The primary coil A of the potentiometer may alternatively be connected in parallel with an inductive coil in series with the resistance s. For adjusting the phase of the voltage induced in the potentiometer secondary coil D, Fig. 2, the primary coils A, B are carried by a common axle with their axes perpendicular. Each coil may be replaced by two coils wound heterocheirally and symmetrically disposed about the axle. The coil D also may be wound in two sections. The coils A, B may be oval or beanshaped and each may have a small circular correcting coil rigidly connected to it. In order to improve the constancy of the voltage induced in the coil D, and to ensure that its phase displacement may be proportional to the angular angle through which the coils A, B are turned, these coils may be tilted or moved axially by ring templets. In order to obtain standard values of alternating current, and hence standard reference voltage, two slide wires QZ, PU, Fig. 7, are arranged so that their sliders X, Y run in step. The current to be measured is passed through a heater H close to a thermopile Th, the voltage generated thereby being balanced by that tapped off from the slide wire PU. A battery E maintains a constant current through a rheostat F, which current is adjusted to standard value as described in Specification 294,053, [Class 37, Electricity, Measuring, &c.], the heater H and its equivalent resistance HI being interchanged and the sliders X, Y being set to a standard reading. The heater is then changed back to working position. An alternative method of standardization is shown in Fig. 11 in which a thermo-junction J of three different metals or alloys, such as iron, nickel, and manganin is connected to a galvanometer G and resistances R1, R2 as shown. Zero deflection of the galvanometer G is obtained with a standard value of the current in the heater Z. In order to compensate for changes in atmospheric temperature, the resistance R1 may be made of copper, nickel, or other metal of high temperature coefficient, or a slide wire SW, graduated in temperatures, may be employed. More than one thermo-junction may be employed, heated by the same or different heaters. A further alternative apparatus for standardizing the current is shown in Fig. 14 in which the whole or a known multiple or fraction of the current to be standardized is passed through two heaters #1, #2 of different resistance-temperature coefficients, arranged to heat oppositely connected thermo-couples. Balance is obtained for a definite value of the current. The temperature coefficient may be reduced by a shunt of suitable material across one of the heaters. An alternative means for temperature correction is shown in Fig. 15, in which the current to be standardized is passed through two heaters Z1, Z2, the latter having an adjustable shunt S2. A thermo-couple Th, heated by the heater Z2, is connected to opposite corners of a Wheatstone bridge having alternate arms of resistance material variable and invariable with temperature respectively, such as nickel and manganin. The bridge is adjusted to balance at room temperature. The other two corners of the bridge are connected to a temperature balancer comprising a thermo-couple J heated by the heater Z1 so as to introduce into the bridge a small voltage proportional to the change of room temperature. Specification 244,596, [Class 37, Electricity, Measuring &c.], is referred to.