220,963. Western Electric Co., Ltd., (Assignees of Harden, W. H., and Graefenecker, M. A.). Aug. 25, 1923, [Convention date]. Telephone systems; automatic exchange systems and apparatus.-A transmission test set and a calling device are connected to the trunk to be tested, automatic switching apparatus is controlled over this trunk by the calling device to select a predetermined test trunk leading back to the test set, and the circuit so set up is held while the desired tests are carried out. For testing a trunk from an automatic to a manual office, the numbers of two interconnected test lines at the manual office are indicated to the operator on call indicators operated by the calling device, and the operator connects the trunk on test and a return trunk over these test lines. In the arrangement shown in Fig. 1, the test set TMS and the impulse sender DB are connected over a jack J to a repeater 49 extending to a trunk 70, 71 to be tested, and over a jack J1 to another similar trunk 86, 87, which during the test is disconnected from its first group selector 99 and connected to a test trunk 92, 93, terminating in the bank of a connector 79. The key K is thrown to position 38 in which a circuit is closed through the sender interrupter springs 42, 43 and a retardation coil 46, for a line relay 48, which places a bridge across the trunk 70, 71. The number of the test trunk 92, 93 is now dialled and the trunk 70, 71 is extended over first and second group selectors 59, 69, the connector 79, and test trunk 92, 93 to the trunk 86, 87. The key K is then thrown to its other position 32, in which the connection set up is held over the retardation coil 47. The keys CCK, CK are operated and alternating current from a source 16 is induced in a circuit including an ammeter 13 and a variable resistance 11. The ammeter reading when all the resistance 11 is in the circuit, is noted for use as a " constant reading " and the calibrating key CK is released. Alternating current is now induced in the circuit including the trunk 70, 71 to be tested, the test trunk 92, 93, and the return trunk 86, 87, and the resistance 11 is varied until the ammeter reading agrees with the " constant reading." The loss, in any desired unit of transmission, of the circuit under test may now be read from the scale of the resistance 11. Similar observations may also be obtained when the repeater 49, or the connector 79, or both, are eliminated from the circuit. When the tests are completed, the plug 96 is removed from the jack J and inserted in another test jack to test another trunk. Any switch or series of switches may be similarly tested by inserting the plug 97 into a jack connected to a test line which extends from the bank of the switch, or the last switch of the series of switches, to be tested, and which is reached by dialling the appropriate digits. The modification shown in Fig. 3 illustrates the method employed for testing a trunk 330, 331 to a manual exchange C. The numbers of two interconnected test lines TL3, TL4 are dialled in turn by the tester over the trunk 330, 331 on test and a return trunk 332, 333, to call-indicators temporarily associated with the repeaters RC, RC1 in which these trunks terminate, and the manual operator completes the return test circuit over the test lines TL3, TL4, which appear in the multiple before her.