885,558. Substation equipment. PARKINSON COWAN Ltd. April 22, 1960 [March 17, 1959 ; April 29, 1959], Nos. 9194/59 and 14610/59. Class 40 (4). A substation comprises a recording medium containing a plurality of pre-recorded messages each of which relates to a possible value of a physical variable which is required to be monitored, and means responsive to an incoming call to set reading means to read off the message appropriate to the value of the physical-variable obtaining at that time. The equipment shown is designed to transmit in succession messages relating to two physical variables. Incoming ringing current operates RR and R follows to bring up T. The latter operates P and S. T then releases R which in turn de-energizes T which releases slowly. The subsequent release of P energizes the sequence switch magnet SS and when S releases, SS is de-energized to step the switch to position 2. In this position B energizes and locks and enables the recording head amplifier. Relays IA and NA also operate the former to connect up a fixed reading head, and the latter to remove a shunt from the telephone lines and to energize the motor MV. The latter drives a drum having a plurality of recording tracks, the first of which carries a station identity announcement and is read by the fixed head so that the identity is transmitted to line. When the drum has rotated through 340 degrees, a cam-operated contact MD1 closes to operate P and 10 degrees later MD1 opens again to release P. The operation of P operates as before to step the sequence switch SS to position 3. In this position IA, NA release and W and IV operate. The former pair disconnect the fixed reading head and stop the motor MV. The latter pair enable a comparator device of the Wheatstone bridge type and connect it up to the potentiometer VPR1 whose position indicates the value of the first physical variable to be monitored. The other pair of arms of the bridge is the potentiometer DPR whose arm corresponds to the position of a shaft driving a movable reading head laterally across surface of the drum. Accord-. ing to the direction of unbalance of the bridge relay YA or YB operates to drive the motor MH which rotates the shaft in such a direction as to balance the bridge. The rotation of the shaft has now positioned the movable reading head opposite the appropriate message which indicates the identity of the variable and its value. The drive for the reading head includes a Geneva stop mechanism which ensures that the head is not positioned between recording tracks. When YA or YB release consequent upon the bridge balancing. When YA or YB operated this operated relay P and when the. bridge is balanced, P releases again and the switch SS is stepped as before to position 4 where relays VA, NA operate the former connecting the movable reading head to the amplifier and the latter energizing the drum motor MV and the message is transmitted. At the end of the rotation of the drum the sequence switch is stepped to position 5. In this position NA and 2V operate and the latter connecting up potentiometer VPR2 and operations proceed in a similar manner for the second variable in positions 5 and 6 of the sequence switch. The variable resistances in the potentiometer circuits are so chosen that the reading head is directed to different tracks on the drum for the two When the sequence switch reaches position 7, relay Z operates to home the sequence switch and disable the equipment. If the handset is raised at any time the cradle switch contacts operate Z to restore SS and disable the monitoring equipment. Reference is also made to the provision of the following features which are not, however, described in detail. (a) Control signals of different frequencies or coded combinations thereof may be sent from a central station to the substation for controlling equipment at the substation. (b) The substation, on an alarm condition arising, may initiate a call to the central station, either by dialling in the case of an automatic exchange, or by emitting verbal destination signals for the benefit of an operator at a manual exchange or by a combination of both methods. The emission of such verbal signals may be caused to cease on reception of a signal of prescribed frequency from the central station. (c) The equipment may also be used to signal the " on-off " condition of devices at the substation. (d) Two moving heads on a drum may be used, the positioning of one head taking place whilst the other is reading. (e) The messages may be in frequency code form and may be logged by automatic apparatus at the central station, the various substations being called by automatic dialling apparatus set in operation by a time switch. (#) The link between the central station and the substation may be a radio-link. The invention may be applied to the monitoring of gas holders and control of gas flow thereat.