28,322. Cohen, B. Dec. 23. Telephone systems; telephone calls.-In an exchange system, the ringing of a subscriber's bell takes place automatically when the exchange operator inserts a plug into the jack, the ringing- current being cut off directly the subscriber's telephone is lifted from the hook. In the system shown in Fig. 1, the insertion of the plug P into the jack closes the circuit of the battery B through the relay F, bush K<1>, resistance coil R, and earth. The tongue M of the relay F then puts the line wire l<1> and the short spring of the jack into connection with the generator G through the resistance lamps T, thus ringing the bell b through earth return. Owing to the connection of the line-wire l<2> to earth at c', no current from the generator passes through the relay C. While the subscriber's bell is ringing, the battery B lights up the signal lamp S, the circuit comprising the tongues L, H and an earth return. As soon as the subscriber removes his telephone from the hook, the earth return to the generator is broken, but the generator circuit is now complete through the lamps T, tongues 0, M, line l<1>, subscriber's instrument, line l<2>, tongue V, and relay C. The tongues H, Y, O are consequently attracted, and the circuit of the battery B closed through the tongue H and relay C, thus keeping the relay magnet energized until the relay F is rendered inactive by the removal of the plug from the switchboard. The generator circuit is broken at O<1>, and the tongues V, O put the lines l<1>, l<2> into connection with lines E, which serve as the medium of communication with the exchange operator or another subscriber. Shock to the operator is prevented by the fact that the plug is not connected with the ringing circuit until it is inserted in the jack. The plug is normally connected through the tongue M to the test winding of an induction coil N. In the modification shown in Fig. 2, a double-tongued relay with independent windings d, s replaces the relay C, and the tongue L is dispensed with. The ringing-circuit is closed through the lamps t, tongues o, m, line l<1>, and earth when the plug is inserted. When the subscriber's telephone is lifted from the hook the return circuit to the generator is made through the line l<2>, winding d (shunted by a non-inductive resistance r), resistance w, and earth. The tongues o, h being attracted, the coil s is energized by the battery b through the tongue a, bush k<1>, and earth, and the speaking-circuit is completed through the lines l<1>, l<2>, e. The non-inductive shunt r prevents impedance to the speaking-currents.