647,764. Telegraph exchange systems. WESTERN ELECTRIC CO., Inc. Oct. 1, 1946, No. 29236. Convention date, Oct. 5, 1945. [Class 40 (iii)] In a printing-telegraph manual exchange system in which a supervisor's monitoring cord circuit may be connected into a switching operator's cord circuit, means is provided for electrically locking the supervisor's teleprinter in an inoperative condition during a line signal of prolonged duration. When the supervisor's teleprinter 302 is plugged in to the supervising cord circuit jacks 207, 216, then over contacts of relays 211, 213, both normal, a battery 214 operates a control relay 304 over its lower bias winding B to its marking contact M, thus locking the receiver selector magnet 305 and preventing whirling of the cam-shaft. When the supervisor plugs in her cord circuit to an operator's cord circuit at jacks 108, 110 and the operator inserts her plugs 104, 111 to answer a call, this condition is undisturbed. The operator's cord circuit is of the type described in U.S.A. Specification 2,222,672, and to answer a call the operator throws a typing key 131 which energizes relays 132 and 134 in parallel and the latter extends the sleeve circuit of the answer plug 104 to further relays (not shown) which operate a relay 133 unless the call has already been answered by another operator at a multipled jack. Relay 133 breaks the connection to these further relays and instead operates relay 105 which in turn locks relay 133. Having answered the call the operator releases the typing key 131 but relay 133 remains operated until the answer plug 104 is withdrawn at the end of the call. When the supervisor desires to supervise the call she operates her monitor key 204 to either the " answer " or " call " position A or C and relay 211 operates over the lowest key contact, and relays 212, 213 then operate in turn. The current in the bias winding of control relay 304 is then reversed and the relay tends to operate to its spacing contact S but the subscriber's negative battery on the ring of plug 104 (or 112) provides, over contacts of relays 133 operated and 132 normal, an over-riding marking current in the upper or line winding L of the control relay 304 and the selector cam-shaft remains locked, until transmission commences. Recall signals. When a local subscriber sends a recall signal earth is momentarily connected to the tip of plug 104 (or 111) and over contacts of 133 (or 126) operated operates relay 119 (or 124) which in turn operates relay 120 (or 125) which locks. When the subscriber terminates his recall signal relay 119 (or 124) releases and a circuit is completed from earth over rotary interrupter 121 to energize a supervisory relay 220 (or 221) which therefore flashes a service supervisory lamp 218 (or 219) and the supervisor then throws the key 204 to the answer A or call C position according to which lamp is flashing if this has not already been done. The operator's lamp 122 (or 127) is also flashed. The flashing of the supervisory relay 220 (or 221) may release relay 211 but not 212 nor 213 and current still flows in the bias winding B of the control relay 304 but the normal line condition holds the relay in its marking position by the current in its line winding L. A recall signal over a repeatered line is a long break and has the same effect as a disconnect signal, described below. Disconnect signal. This is received as a continuous earth on the tip conductor which operates relay 119 (or 124) causing the operator's supervisory lamp 122 (or 127) to glow and operating relay 220 (or 221) to light the service supervisory lamp 218 (or 219) and release all the relays 211 ... 213. The contacts of these relays (Fig. 3) reverse the current in the bias winding B of the control relay 304 which operates to its mark contact M and locks the selector magnet 305 to prevent whirling of the selector cam-shaft.