14,317. Aitken, W. June 19. Exchange systems and apparatus; intercommunication systems and apparatus.-In an intercommunication system having an exchange line, the plugs at the several stations are controlled by an electromagnetically-actuated latchplate. If the desired station is idle when the plug is inserted, an electro-magnet is energized to cause the latch-plate to lock the plug in position until the connexion is broken at the end of the conversation. In the case of the desired station being busy an initial contact which is closed by the plug energizes the electromagnet and prevents its armature from engaging with the latch-plate. In this case the plug is not locked and the circuit of the speaking instruments is held open. In the system described there are three intercommunication stations A, A<1>, A<2>, the exchange calls being received by the station A on the bell 61. A station as A<1> is provided with two intercommunication jacks AJ<1> and A<2>J<1> and an exchange jack XJ<1>. The construction of a jack and plug is shown in Fig. 1. The plug P is arranged to slide between guide rails over the plug holes 2. When station A wishes to call station A', the operator moves the plug to the jack A<1>J and pushes the head 39 through the apertures 2, 5. If station A<1> is idle, the full insertion of the plug causes contact to be made between the springs 8, 9 and 11, 12, and a circuit for the battery B is made over the lead 66, winding 67, contacts 8, 9 of jack A<1>J, receiver 62, microphone 68, contacts 16, 17, contact 69, contacts 12, 11 of jack A'J, winding 70, back to the battery. The energizing of the magnet E causes the pin 20 on the armature 19 to engage with the hook 21 on the latch-plate 3 and to raise the latch-plate so that it engages with the notch 44 in the part 38 of the plug and holds the plug in position. The movement of the latch-plate caused by the electro-magnet E also moves the spring 16 from the contact 17 to the contact 18. The depression of the ringing key e now places the bell 74 in a circuit of the battery B, the bell being biassed against the currents which previously reached it over the microphone 68 and receiver 62. When the receiver 75 is lifted at the station A<1>, it is placed in the circuit of the battery. B from spring 7 at station A through receiver 75, microphone 76, contacts 73, 77, contacts 64, 65 back to spring 10 at station A. If station A<1> is busy when station A calls and the contact 64, 65 therefore closed, the first contact between the springs 7, 8 and 10, 11 completes a circuit from the battery B over the lead 66, winding 67 of magnet E, contacts 8, 7 of jack A<1>J, receiver 75 at station A<1>, microphone 76, contacts 73, 77, contacts 64, 65, contacts 10, 11, and winding 70 back to the battery. The energizing of the magnet E causes the armature 19 to rise without the pin 20 engaging with the latch-plate 3, and hence when the plug is fully inserted, it is not locked in place and the latch-plate is not raised to bring the spring 16 into engagement with the spring 18, the instrument circuit therefore remaining open. At station A<2> additional contacts are provided for the relay E<2> for the purpose of separating the calling line from the speaking connexions, and inserting the bell in a separate circuit for receiving a call during conversation. An extra calling battery B<1> may be used in order to avoid biassing the bells. When a call is received from the exchange line, the operator at station A moves the switch S so as to close the contacts 90, 91 and 92, 93, thus connecting the line to his instruments. When he has ascertained what connexion is desired, he switches in the holding-coil 60 and calls up the wanted station. The exchange line is now connected to the desired station by operating the switch S to close the contacts 95, 96 and 97, 98, this movement also resetting the holding key 94. The coil 99 holds the switch S in this position until the conversation is finished. When a station, as A<2>, wishes to call the exchange, the operator first calls station A, and if the exchange line is idle, the operator at this station connects the exchange line by the switch S, while the operator at station A<2> moves his plug to the jack XJ<2>. Additional contacts are provided on the jacks XJ<1>, XJ<2> to provide the holding circuit for the magnets E<1>, E<2>. Specifications 12,974/12 and 12,975/12 are referred to.