670,251. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. Oct. 8, 1948 [Oct. 9, 1947], No. 26286/48. Class 40 (iv). An automatic exchange consists of links terminating at either end in switches each outlet of which is connected to a switch having outlets to a different group. Thus, in a 10,000 line exchange, Fig. 1, a link c has a switch y<SP>2</SP> set by the thousands and hundreds digits of the called line e each outlet being connected to a switch y<SP>1</SP> serving a particular hundreds group and set by the tens and units digits. Similar switches x<SP>2</SP>, x<SP>1</SP> give the link access to the calling line d. Inter-exchange trunks may be connected over a part b<SP>1</SP> of the arc y<SP>2</SP> or may be allotted " half-links " having access to switches x<SP>1</SP>, Fig. 2 (not shown). A register E when seized marks all the switches y<SP>1</SP>, x<SP>1</SP> giving access to the called and calling lines respectively, whereupon all free paths between them are marked and a link finder associated with the register hunts for a link included in one of the free paths. By this means free hunting of the selector switches is avoided. The calling line is initially connected to the register through a switch x<SP>1</SP> acting as a normal finder and thence via a secondary line finder which may or may not be a switch x<SP>2</SP> of one of the links. Marking arrangements. In the arrangement shown in Fig. 5 the switch x<SP>2</SP> of the link C is used as the secondary line finder and the link is connected to the register over switch D for the purpose of effecting the marking on the calling line side and for the reception of the wanted party's number. The switch c<SP>1</SP> is set by the thousand and hundreds digits and applies a marking to a terminal a associated with all the final selectors E<SP>1</SP> serving the wanted hundreds group. Each switch has a busy relay OE<SP>1</SP> which when the switch is idle extends the terminal a to a point a<SP>1</SP> individual to each final selector. Each terminal a<SP>1</SP> is connected to terminals a<SP>3</SP>, associated with links having access to that particular final selector, over the double resistance rr. Thus each link having access to the wanted subscriber is marked on its terminal a<SP>3</SP> and the mid-points a<SP>2</SP> of the resistances rr are also marked to indicate free final selectors having access to the wanted group. Corresponding markings are also applied to the calling line side of the links via switches D, x<SP>2</SP> and the point b to mark the points b<SP>2</SP> and also the points f in each of the links in which the busy relay OC<SP>1</SP> is unoperated. Setting of the switches, Fig. 6. The calling subscriber having obtained access to the register E over and switch x<SP>1</SP>, switch x<SP>2</SP> of link C finder D (which may represent two finders like F<SP>1</SP>, F<SP>1</SP>) the thousands and hundreds digits of the wanted line are registered on switch c<SP>1</SP> and the tens and units digits on switch c<SP>2</SP>. A marking signal is now applied across terminals p<SP>1</SP>, p<SP>2</SP> and this extends back to the terminals a<SP>3</SP>, f of all links in free paths. The finders F<SP>1</SP>, F<SP>2</SP> then hunt for a link c<SP>1</SP> so marked and when such a link is found relay R operates to stop the finders. The marking signal is now applied between terminals p<SP>2</SP> and k and switch y<SP>1</SP>2 is caused to rotate and stop when the marking circuit is completed over terminals a2, a. The marking signal is then applied across terminals k, h to set the final selector y<SP>1</SP>1. Switches x<SP>1</SP>2, x<SP>1</SP>1 are set in a similar manner using terminals p<SP>1</SP>, p<SP>3</SP> in the register. The register E and the link C are now released and the connection completed and held under control of the link c<SP>1</SP> in known manner. Marking signals. Since the exchange may require more than one register, each must use a characteristic marking signal. This may consist of D.C. applied at a moment in a cycle characteristic of the register. Rectifiers or like decoupling devices may then be included in the resistances rr to prevent false marking of busy paths. Alternatively the marking signal may consist of a frequency characteristic of the register appropriate filters being inserted both in the frequency source and in the relay R.