Patent GB-917196-A (GB)
Title: Improvements in or relating to electrical measuring apparatus
IPC: G01R1/28
Filed: 1960-03-10
Abstract: 917,196. Resistance, impedance, measuring- WAYNE KERR LABORATORIES Ltd., and MILDWATER, J. June 12, 1961 [March 10, 1960], No. 8538/60. Class 37. Apparatus for measuring the magnitude of an unknown electrical quantity by indication on a calibrated indicator wherein step adjustable means balance the unknown against a known standard variable so that the indicator displays the magnitude of the difference between the unknown and the balanced major part thereof, and wherein means are provided for increasing the sensitivity of the indication as a relation between the display of the indicator and the magnitude of the difference, when the stepadjusting means is operated. Fig. 1 shows an impedance 15 [Zu] connected between the input of high-gain amplifier 16 and wiper 14 of the decade tapped secondary 12 of A.C. energized transformer 10; the zero tap being returned to the amplifier output, which is measurable by meter 19. A standard impedance 20 [Z 8 ] is connected between the amplifier input and the wiper 21 of a decade switch ganged to 14 such that the zero tap is connected over wiper 22 to tap 24 of secondary winding 13 providing a standard datum voltage; the winding having other taps 25, 26, 27 providing voltages of X10, X100 and X1000 datum, respectively. Winding 13 is returned to amplifier earth and wiper 23 ganged with 22 engages above-zero taps 25, 26, 27 and a blank contact respectively. It is shown that initially with switches 14, 21 and 23, 24, as shown, E/Z 8  =E 0 /Z u   z 8  Z u  where E is the datum voltage between adjacent taps of 12 and selected from tap 24 and E o  is the output meter reading giving an approximate indication of Z u , so that if switch 14 is set to a tap representing the first significant figure of Zu and switch wiper 22 is placed in its second position the meter reading precisely indicates the remaining two significant figures. With wiper 22 as shown, movement of wipers 14, 21 from zero connects impedance 20 over wiper 23 to tap 25, corresponding to the second position of wiper 22, to apply a voltage of X10 datum thereto. If the first significant figure is zero, wipers 22, 23 may be advanced one step to increase sensitivity of indication ten times, after which switch 14 is set to the next significant figure and the meter reading indicates subsequent significant figures. A corresponding further advance of wipers 22, 23 may be made if the first two significant figures are zero, when the succeeding significant figures will be read by the meter. The switching for obtaining X1 X10, X100, X1000 datum voltages from transformer 13 for application to the standard impedance may be replaced by a standard datum voltage source and a standard impedance switchably reducible in steps corresponding to factors of 10 to values of 1, 1/10, 1/100,1/1000. In a modification (Fig. 2) the tapped winding 12 is returned to the amplifier output over a second decade tapped winding 31 and wiper 30; the winding having tapped voltage steps each 10 X the corresponding tapped voltage step of winding 12. Wiper 30 is ganged with wiper 32 so that in its zero position standard impedance 20 is connected to wiper 21, but in its off-zero positions the impedance 20 is connected to wiper 33 ganged with wipers 22, 23 to traverse contacts connected to taps of winding 13 so as to derive X100, X1000 datum voltages when switch wiper 30 has moved from its zero position. Initially wipers 14, 30 are zero set and wipers 22, 23, 33 set in their first positions. Meter 19 reads the unknown impedance and wiper 14 is set to the first significant figure. If this is zero, sensitivity is increased 10X by moving wipers 22, 23, 32 one step and repeating the operation. The first significant figure of the remainder figure on the meter is noted and wiper 30 set to this figure; wiper 32 also being set to increase the sensitivity by 10X, so that the first two significant figures of the unknown impedance are indicated by positions of wipers 14, 30 and the remaining two significant figures by meter 19. In a further modification (Fig. 3, not shown), a high-gain amplifier feeding a meter has a first standard feed-back impedance from output to input and an auxiliary standard impedance also connected to the input from a first decade switch tapping the first secondary of an A.C. energized transformer having a second secondary applying datum voltage to the first tap and 10X datum voltage to the remaining taps of a second decade switch ganged with the first, which is connected through the unknown impedance to the amplifier input. Initially the meter indicates the ratio of the unknown and the standard impedances, and the auxiliary standard impedance is connected to the tap of the first secondary which corresponds to the first significant figure, after which the meter indicates the remaining significant figures.