832,684. Resistance measurement. GENERAL ELECTRIC CO. May 25, 1956 [May 26, 1955], No. 16277/56. Class 37. [Also in Group XXXV] In apparatus for measuring a low ohmic resistance, e.g. of a winding on a magnetic core, such resistance is connected in series with the primary of a saturable reactor and a constant D.C. source saturating such reactor; a secondary winding of which is energized by an alternating modulating current which is rectified to a D.C. proportional to the current in the reactor primary and passing through a calibrated resistor, to develop a voltage drop which is compared with the voltage drop across the low resistance to produce an indication of the ohmic value thereof. Construction.-In Fig. 4 a winding 10 of a high power transformer comprising series inductance and resistance is D.C. energized from a constant current amplidyne generator 13 in series with resistance 61 and conductor 62 forming the primaries of saturable reactors 20, 21 wound on cores having rectangular hysteresis loops, in the position shown of three position switch 60; the remaining positions of the latter alternately completing the excitation circuit through resistances 61, 63 and conductors 62, 64 giving two traversals of the reactor magnetic circuits, and completing the excitation circuit through resistances 61, 63, 65 and conductors 62, 64, 66 giving three traversals thereof; so that in each switch position the same magnetomotive force excites the reactor cores for three distinct values of range current. The secondaries 22, 23 are connected in series with a bridge rectifier 24 and a tapped autotransformer 25 energized from an A.C. fed supply transformer so that on successive half-cycles the A.C. and D.C. fluxes oppose in the core of reactor 20 and aid in the core of reactor 21, and vice versa. The bridge rectified net reactor secondary current (dependent on the amplidyne output current traversing the test transformer winding) energizes a resistance 30 in series with a resistance shunted slidewire potentiometer 31 and a filter choke 32, and the slider is shunted to the free end of resistance 30 by a high value chain of variable resistances 68, 69, 70, 71, 72, 73 of which 68, 73; 69, 72; and 70, 71 are respectively ganged for adjustment by decade dials so that the total resistance remains constant. The transformer winding 10 is shunted by large resistances 37, 38 of which three corresponding alternative values are selected by switches 78, 79, and the unidirectional voltage appearing between the junctions 38, 37 and 70, 71 is applied through a series variable resistance 74 ganged for adjustment with 68, 73 to a filter 76 feeding a recording potentiometer detector circuit controlling a motor 40 adjusting potentiometer 31 for null and driving a pointer 41. The amplidyne excitation is controlled by a variable unidirectional voltage derived from voltage regulating tube 86 by a potentiometer 90 in series with variable resistances 89, 91 and applied to series resistances 92, 93 grounded over resistances 96, 65, 63, 61; the voltage at junction 92, 93 being D.C. amplified by triodes 85, 101 and push-pull tetrodes 103, 104 energizing the amplidyne field control windings. Negative feedback for stability is applied from the amplidyne output to the input of triode 85 over resistance-capacitance filter 105, 106 and series resistance and capacitance 107, 108, while a control voltage proportional to the amplidyne output current is opposed at the junction 97 of resistances 92, 93 to the control voltage from potentiometer 90, so that the output current is maintained constant at a value proportional thereto. Voltmeter 98 indicating the output current is shunted across resistances 61, 63, 65, and milliammeters 131, 132 indicating the excitation values are connected in series with the field windings. Over-voltage amplidyne field winding 111 is connected across the output over normally closed contacts 112 of relay 115 while the cathodes of pentodes 103, 104 are returned to ground through normally open contacts 120 thereof, the relay being energizable by A.C. energized gas tetrode 114 biased from potentiometer 117 and controlled by a voltage derived from resistance potentiometer 94, 95 across resistances 92, 93. Further series windings 125 of the reactors are energized by a standing direct voltage across resistance 87 in series with regulating tube 86 to compensate for the necessary excitation current of the reactors so that their secondary currents are directly proportional to their primary currents. Operation.-Initially the control voltage from potentiometer 90 biases tetrode 114 to strike and open contacts 112 while closing contacts 120, so that the output of the amplidyne rises to a level controlled by the potentiometer and current range switch 60. Any voltage overshoot causes the voltage at the junction of resistances 94, 95 to fall due to voltage fed back from resistance chain 61, 63, 65, 96 and cuts off tetrode 114 releassing relay 115, closing contacts 112, and opening contacts 120. Field winding 111 is then energized to heavily reduce the output voltage. When stability is attained, the amplidyne output current is held controlled to a value set by potentiometer 90 in conjunction with the fed-back control voltage. Selector switch 60 is adjusted for saturation of the cores of reactors 20, 21, and appropriate resistances 37, 38 are selected by range switches 78, 79 for the expected level of winding resistance to be measured. Variable resistances 68, 71; 69, 72; and 70, 73 are manually adjusted until the slider of potentiometer 31 and associated pointer 41 are set to mid-range by motor 40 to zeroize the total voltage applied to filter 76. It is shown that the total reading of the decade dials of the variable resistances is then proportional to the resistance of the transformer primary winding and may be calibrated directly therein. A fine correction reading for addition to that obtained above is given by the deviation of the slider arm and pointer of potentiometer 31 from its central or zero position for null, as indicated by pointer 41.