Patent GB-254164-A (GB)
Title: Improvements in electrical testing devices
IPC: H04M3/30
Filed: 1925-10-26
Abstract: 254,164. Western Electric Co., Ltd., (Bell Telephone Laboratories, Inc.). Oct. 26, 1925. Measuring the transmission equivalent of telephone lines. - The measurement is effected by automatically operating testing means. A cord circuit 3 at station A is connected to the line L to be tested and to one of a series of multiple jacks 2 connected to an oscillator 1, which may have a frequency of 1000 cycles. At station B a cord circuit 4 connects the line L to one of a number of contacts 5<1> of a finder switch 5. Current from the oscillator 1 is then transmitted over the line L and conductor 6 through a transformer to the measuring set 7, which comprises a device 8 for introducing loss step by step and a device 9 consisting of a potentiometer resistance r<11> for decreasing the loss step by step. The current is then transmitted through an amplifier rectifier 12 to a relay 13. When the devices 8, 9 are in their normal positions, i.e. when the switch 10 is in its left-hand position and the switch 11 in its lowest position, the relay 13 operates and closes its contact 14, which completes the circuit of a stepping mechanism 18 which moves the switch 10 so as to add resistance elements r in series in the circuit and decrease the number of resistance elements r' connected in shunt across the circuit, thus decreasing the current transmitted until the relay 13 releases, opening circuit 14 and closing contact 15. This closes the circuit of a stepping-mechanism 19 which moves the switch 11 of the potentiometer resistance r<11> upwards so as to increase the voltage across the input terminals of the amplifier 12 until the current is just strong enough to operate the relay 13, which opens the contact 15, stopping the mechanism 19, and closes the contact 14 which locks up all the apparatus. The potentiometer device 9 is so proportioned that the gain in current strength due to its operation throughout its range is equal to the loss of current strength due to one step of the device 8. Associated with the devices 8, 9 is a signal panel indicating the number of transmission units and fraction thereof which have been cut into the line to obtain the value of the current that just operates the relay 13. The current in. put from the oscillator 1 and the current, required to operate the relay 13 being known, the total attenuation is known. By subtracting from this the attenuation indicated as having been introduced into the measuring set 7, the attenuation of the line L is determined. The connections of the measuring set are shown in detail in Fig. 2. The conductor 6 terminates in a plug P into which fits a jack J, from which the line current is transmitted through a transformer to the resistance device 8 for introducing loss and the potentiometer 9 for introducing gain, and is then applied to the input terminals of the amplifier rectifier 12 and thence to the relay 13. In the normal position of the devices 8, 9 the transmitted current is strong enough to operate the relay 13 which closes at 14 the circuit of the relay 22, which operates and thus opens a circuit 31 common to all the indicating-lights in the signal panel, closes the circuit of relay 23, which locks up, and closes the circuit of relay 35 which operates intermittently and steps round the resistance device 8 until the transmitted current is too weak to operate the relay 13. This then opens, releasing relay 22 but leaving relay 23 locked. The relay 22 opens the circuit of the relay 35 stopping the action of the device 8. The relay 13 also closes at 15 the circuit of a relay 25, which opens the circuit 31, closes the circuit of a relay 24 which locks up over its left-hand contact and a contact of the relay 23, and closes the circuit of a relay 42 which operates intermittently to step round the potentiometer device 9 until the transmitted current is just strong enough to operate the relay 13, which closes contact 14. The circuit of the relay 22, however, is broken at the locked relay 24. The breaking of the contact 15 releases the relay 25, which opens the circuit of the relay 42 and completes the circuit 31, thereby lighting those lamps on the signal panel corresponding to the positions of the switches 8, 9. When the measurement has been obtained, the plug P is withdrawn from the jack J, releasing a relay 21. This releases the relay 23. which releases the relay 24. The release of the relay 21 also completes circuits 45, 46 to the fourth banks of the selector mechanisms 8, 9 through the windings of relays 26, 27. The selectors thereupon step themselves round to normal, the relays 26, 27 at the same time operating and preventing another measurement from starting until the apparatus is restored to normal. When this occurs, the contact through the fourth bank of each of the selectors 8, 9 has been broken, the relays 26, 27 release, and the circuit is ready for another measurement.