Patent GB-190614642-A (GB)
Title: Improvements in and connected with Secret Keyboard for Central Exchange Switchboard.
Inventor: BEARD RICHARD MEEK
Filed: 1906-06-27
Granted: 1907-06-27
Abstract: 14,642. Beard, R. M. June 27. Telephone systems. - A secret keyboard is arranged to prevent the operator from listening into a talking circuit. A common n battery cord circuit is equipped with a listening-key which opens circuit towards one cord while the operator's telephone is connected to the other, but this key does not open the direct path of the battery currents between the two cords, thus avoiding a disconnect signal to the second board when the operator desires to cut in on the calling- subscriber's circuit. A locking - mechanism ensures that only one key in the keyboard is in listening - phase at the same time, and a sealed compartment contains all the apparatus and circuits. When subscriber S raises his receiver from its hook, relay 4 closes momentarily a local circuit through lamp 3, whereupon the operator places plug 8 in jack J, thus energizing relay 9 which attracts armatures 5, 6, cutting off the subscriber from his line signal, and operating the supervisory relay V. The operator now moves listening-key 9, wedging apart the blades 26, 27 by the circular plug 25, which is pivoted between the blades, and making connexion at points 29, 30, thus establishing a talking circuit. Having ascertained what connexion to make, the operator places plug 8<1> into the jack J<1>, and restores the key to its normal position, connecting the subscribers through cords M, N, contacts C, and condensers 35, but if the operator attempts to listen the subscribers are immediately disconnected. In practice, a busy test is always made before connecting with a called subscriber in the following way. Wire 20 is connected to the high-potential side of the battery and wire 23 to the low-potential side, and condenser K has therefore a charge corresponding to the difference of potential. If a called line is busy, the corresponding jack J<1> is connected to the high-potential side of the battery, and when the operator touches tip 22<1> to contact 17<1>, the potential of wires 28, 23 is raised and condenser K discharges, producing a click in the operator's telephone receiver. The test is thus made without movement of the operator's key or special manipulation. Plug 8' being inserted in jack J' as described above, the supervisory lamp 11<1> is illuminated and continues to glow until subscribers S' removes his receiver, when the lamp circuit is broken, and the busy test circuit is also broken owing to attraction of armature 12<1>. Meantime the operator moves wedge 25 into a central position, placing the subscribers in communication. When the subscribers have finished talking, lamps 11, 11<1> indicate the fact to the operator, who removes the plugs from the jacks. To prevent an operator from so manipulating the keys of two cord circuits as to connect them to her own instrument, a mechanical interlocking-device is used, as shown in Figs. 2 and 4. A double wedge 37 is secured to the operator's lever 36 which pivots about the point 38, and a portion 25 forces apart the blades 26, 27 or 26<1>, 27<1>, according to the direction in which the key lever 36 is thrown. The wedges have concave portions 41, and antifriction rollers 44 having the same curvature are attached to the extremities of bearers 42, which are short bars guided to move longitudinally by bearings 43. The keys Q are arranged side by side on the keyboard and the bearers abut against one another in their normal position, but when a wedge is moved in one direction or the other, faces 39, 40 force apart the rollers 44, displacing all the bearers and locking the remaining levers 36. To prevent the operator from making connexion within the keyboard cabinet, the lid of the cabinet is sealed.