 for security, see Section 31.7, analyses the digits received and selecting sets the appropriate crosspoints to establish the call. Being common, To other finger the control equipment can be used to set up only one connection at crosspoints Select bar a time but operates SO quickly that this restriction is not noticeable in column to customers. The director, as used in the Strowger system, is an example of register control. Some of the disadvantages of the Strowger swit- ches have been overcome with other designs of mechanical switch but they are generally unsuitable for direct control from the cus- tomer's dial. Register control provides a way of using such swit- ches. The most successful register controlled systems are based on the crossbar switch which is mechanically simpler than the rotary Pivot switches of the Strowger system and can be designed for greater reliability and smaller size. Translation facilities like those provided by the director system, but not limited by considerations of mech- anical switch sizes, are readily obtained because there is no inherent relationship between number allocation and particular switch out- lets. Select The Crossbar switch mechanism consists of a rectangular array of Hold magnets magnets contact sets or crosspoints, which can be selected by energising the electromagnets corresponding to their vertical and horizontal coor- Figure 31.15 Crossbar switch 31/12 Principles of switching systems finger should be thought of as a stiff piece of wire sticking up out of the paper. The horizontal (select) and vertical (hold) bars move in the direction shown by the arrows when their associated coils are Junctions RELAY energised. It can be seen that the crosspoint contact will be operated only when the horizontal bar 'selects' before the vertical bar oper- SWITCH ates. Once operated, the contact remains held by the vertical bar which traps the finger and holds it even when the horizontal bar has been restored. The horizontal bar can be used again to select another crosspoint in the 