Improvements in or relating to electrical measuring apparatus and to standard resistances for use therewith

摘要

1,147,189. Resistance measurement; electrophysical measurement. WAYNE KERR LABORATORIES Ltd., and J. MILDWATER. May 24, 1965 [May 26, 1965], No.22425/65. Headings G1N and G1U. An arrangement for providing a perfect short circuit in a part of a circuit providing a continuous current path incorporates a high gain amplifier, to the input terminals of which two points at opposite ends of the circuit part are connected via a pair of leads. A second separate pair of leads connects the two output terminals of the amplifier to the opposite ends of the part, at the same or different points as before, the amplifier being arranged to feed current to the second pair of leads to pass in a direction through the circuit part such as to cancel the voltage on the first pair. In a bridge circuit for measuring low resistance, the connection 15-18 between resistors 10 and 11 being compared, which connects to the bridge output indicator 24, must have negligible resistance. A perfect short circuit is established between the points 15, 18 by connecting them to the input leads 31, 32 of the amplifier 30, whose output leads 33, 34 connect to further points 13, 16 associated with the resistors 10, 11, (which are of the four terminal type). The arrangement may be used for short circuiting, when required, a resistor (68) which is part of a bridge used for measuring the conductivity of a liquid conductivity cell (66, Fig. 6, not shown). The cell is of the type having spaced electrodes whose spacing can be altered by a known amount so that two measurements can be made to enable end effects near electrodes to be eliminated. A fixed resistor (67) balances the bridge for one electrode setting, while a resistor (68) having a value corresponding to a standard impedance change produced by altering the electrode spacing is switched in and out as required by a switch (69), a perfect short circuit being established by an amplifier (70) operating as described above. A further amplifier (71) and indicator (73) respond to and show any small differences between the measured conductivity and the switched standard resistance.