527,505. Semi-automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. (Pouliart, W.). April 4, 1939, No. 10521. [Class 40 (iv)] In a system in which groups of condensers act as registers, all the condensers of a register are charged to one polarity before registration takes place, registration causing this polarity to be reversed in certain condensers and the charge detecting means is so arranged that when it is connected to a condenser charged to the reversed polarity current flows to discharge the condenser and then charge it to the initial polarity. The invention is described with reference to a key-sender comprising digit keys DK1-0 and " prefix " keys PK1, PK2 &c. the operation of one of which. controls the transmission of two or more digits. When the number is completely set-up by the digit keys, the first is registered on a relay register Am-Dm and subsequent ones are registered on condenser groups, the settings of which are transferred to the relay register in turn during the sending. of the number. When the number is partly set up by a prefix key the prefix digits are marked in the sending switch bank by an individual prefix relay and the sequence switch automatically connects up the appropriate register for receiving the next digit from the digit keys. The sender controls a local number indicator of the type described in Specification 517,485, [Group XXXVIII]. Registration of number. When the operator associates her key-set with a cord circuit or trunk by means of a key such as AK, relay B pulls up if the sender is in normal condition, locks, lights lamp DL and energizes D in series with magnet SM of a switch S which combines the functions of digit-distributer and sendercontrol switch. SM is unable to pull up in this circuit but is energized when D is shortcircuited on the depression of any digit key and drives the switch when the key is released. The keys connect battery in different combinations to the wires L1-L4 to energize one or more relays in the register Am-Dm for the first digit and to reverse the direction of the charge in one or more of the condensers in the remaining registers for subsequent digits, all the condensers of the register that is being controlled having just been charged by the operation of D when the key for the preceding digit was released. If one of the prefix keys is operated the corresponding prefix relay, say, Prl, is operated in series with relay Pc, lights a tell-tale lamp PL1 and locks. In the system shown prefixes consist of three digits, but the number may be varied by modifying the jumpering shown in dotted lines at bank sma and to the left of bank smb. On the operation of Pc the switch S races round to position 3 in which D which has hitherto been short-circuited charges the condensers of the fourth register in readiness for the next digit. Transmission of registered digits. On the operation of starting key SK, Rd pulls up and homes the switch S whereupon Ss pulls up, locks, dims lamp DL and connects the sending switch magnet to contact 1 of relay I which with Ph is pulsing at a regular rate by interaction with P as described below. Pick-up relay Is pulls up over the first terminals of the sending switch, locks and enables Ph to send impulses over the outgoing line. In the case of a number set up entirely by digit keys Pc is not operated and the first digit is terminated in accordance with the condition of relays Am-Dm by stop relay Sp which pulls up in parallel with Rs, unlocks the pick-up relay Is and locks itself and Rs over wiper ssmc of the sending switch which now races to position 17. Rs unlocks the relays Am-Dm and energizes magnet SM of the switch S. In position 17 Rs, Sp are unlocked and magnet SSM then receives impulses from the interrupter set until it again reaches position 1. Relay Ss on the restoration of switch S connected the high resistance winding of a key Dc to the lefthand sides of the register condensers, and Rd on the operation of the start key released D and energized Dh which energized Dc. Dc remained locked up and released Dh. When the sending switch makes its first step in counting out the ante-digital pause it connects ground at ssma to the first condenser of the first register. If the charge in the condenser has been reversed by the digit key it now reverses again, the direction of current being such as to de-energize Dc. Dh is then operated and re-energizes Dc but remains energized itself long enough to operate the first register relay Am which locks up. If the charge in the register condenser has not been reversed Dc remains operated and the register relay remains de-energized. The transfer of the setting of the condenser register is completed in subsequent positions of the sending switch. The second digit is then sent out in the same way as the first. When the sending switch reaches normal after the final digit has been sent none of the register relays is energized and accordingly a circuit is made for Es which locks, releases Rd, Ss, homes the switch S, and then falls back. The lamp DL glows brightly again to indicate that sending is complete and that more digits may be keyed if necessary. In case a prefix key is used, the markings for the first three digits are made by contacts of the prefix relay. If subsequent digits have been registered they are then sent as described above. If not, the circuit is retored to normal and they may be keyed when the operator receives the " sending complete " signal. Local number indicator. Numerical impulses are sent to the number indicator over wire Ld by relay I and change-over impulses over wire Le by relay Rs. The number continues to be displayed after the release of the registersender, but when the latter is seized again relay Dr is operated temporarily and releases the number indicator by grounding both the control wires Le, Ld. Impulse generating circuit. When the register-sender is seized condenser CN is charged in series with a resistance and the shunted right-hand winding of polarized relay P to delay somewhat its operation by its other winding which is energized at the same time. Relay P energizes Ph and I in series and the former reverses the direction of the current in the operating winding of P to cause its release, but at the same time removes the shunt from the delaying winding and reverses the charge on condenser CN through that winding to cause this release to be delayed. Ph. I fall again, and the cycle is repeated. The desired impulse ratio is obtained by a suitable choice of circuit constants.