Patent GB-180008-A (GB)
Title: Methods of and means for measuring unbalance of telephone and like lines
IPC: H04B3/46
Filed: 1921-02-11
Abstract: 180,008. Western Electric Co., Ltd., (Western Electric Co., Inc.). Feb. 11, 1921. Measuring unbalance of telephone and like lines.-The degree of unbalance between a real and an artificial telephone or telegraph line is measured by connecting them in series with the secondary windings of a transformer, the primary of which is energized by alternating current of the desired frequency. The current traversing a measuring-circuit connected across the mid-points of the secondary windings is a measure of the want of balance. The measurement is taken by varying the impedance of the measuring-circuit until the current transmitted thereby has a standard value, the unit for the graduation of the impedance-controlling dial being a mile of standard cable. As shown in Fig. 1, alternating current is supplied through a transformer 5 to a circuit containing resistances 6, 7. Across the resistance 7 connection is made to terminals of a switch 8, and when this is in its upper position, current flows to the primary coil 30 of a balanced transformer, the secondaries of which are connected in series with the telephone line L and the artificial line N. The mid-points 31 of the secondaries are connected through the switch 8 to a measuring-circuit M which comprises an impedance or artificial line 9 variable by means of dials 10, 11. By a potentiometer 12 and a transformer, the current influences the grid of an amplifying-valve 17, the output from which is rectified by the valve 13 and produces a direct current through a resistance 15, and the strength of this current is a measure of the unbalance of the lines L, N. In order to provide a " null " method of working, and to prevent variations of amplitude of the source 32 from affecting measurements, current is taken off from the source 32, across a resistance 6, and is applied through a transformer 35 and rectifier 13<1> to a resistance 15<1>. The two resistances 15, 15<1> are joined at one end and a galvanometer 14 is connected between the opposite ends. To prepare the apparatus, the switch 8 is swung down so as to connect the measuring-circuit M directly to the source 32. The dials 10, 11, are moved to the position of maximum impedance and the potentiometer 12 is adjusted until the galvanometer pointer 14 is at zero, indicating equality of current through the resistances 15, 15<1>. The switch 8 is then moved over, and the current passing to the circuit M is then only that due to the want of balance of the lines L, N, while the same current as before passes through the circuit R. The galvanometer needle is thus displaced, and in order to return it to zero it is necessary to reduce the impedance 9 by means of the dials 10, 11. The dials may be graduated so that the reading is the number of miles of standard cable corresponding to the impedance reduction. Thus the higher the reading in miles, the more perfect is the balance. The reading may be regarded as the number of miles of cable that would give a transmission efficiency equal to that of the transmission from circuit 32 to the circuit connected across the points 31. It is found that when the lines are used in connection with certain types of repeaters, the unbalance measured in standard miles and reduced by about seven miles, is equal to the maximum allowable amplification of the repeater stated in miles. As shown in Fig. 2, the apparatus is arranged to deduct the seven miles, so that the reading taken is the maximum amplification of the repeater in miles. For this purpose the circuits of Fig. 2 are substituted for those between terminals a, b, c, d, of Fig. 1. When the switches 43, 44 are in the calibrating-position, impedances l, n, of unequal magnitude are connected to the two halves of the secondary, the inequality being such that the transmission loss between a<1>, c<1> and b<1>, d<1> is equal to that of seven miles of standard cable. The calibration is then effected as before, and the switches 43, 44 moved to connect in the lines L, N. The dial reading corresponding to no galvanometer defletion then indicates directly the maximum amplification that may be given to the repeater.