A good idea of the mechanical construction of this relay may be obtained from Fig. 195.  The entire selecting function of the Roberts system is performed by this simple piece of apparatus at each station.

[Illustration:  Fig. 195.  Roberts Latching Relay]

The diagram of Fig. 196 shows, in simplified form, a four-station line, the circuits being given more in detail than in the diagrams of Chapter XVI.

It will be noticed that the ringer and the relay coil 6 at the first station are bridged across the sides of the line leading to the central office.  In like manner the bell and the relay magnets are bridged across the two limbs of the line leading into each succeeding station, but this bridge at each of the stations beyond Station A is ineffective because the line extension R_{x} is open at the next station nearest the central office.

[Illustration:  Fig. 196.  Simplified Circuits of Roberts System]

In order to ring Station A it is only necessary to send out ringing current from the central office.  This current is in such direction as not to cause the operation of the relay, although it passes through the coil 6.  If, on the other hand, it is desired to ring Station B, a preliminary impulse would be sent over the metallic circuit from the central office, which impulse would be of such direction as to operate the relay at Station A, but not to operate the bell at that station.  The operation of the relay at Station A causes the spring 2 of this relay to engage the spring 3, thus extending the line on to the second station.  After the spring 2 at Station A has been forced into contact with the spring 3, it is caught by the latch of the spring 5 and held mechanically.  When the impulse from the central office ceases, the spring 1 resumes its normal position, thus breaking the bridge circuit through the bell at that station.  It is apparent now that the action of coil 6 at Station A has made the relay powerless to perform any further action, and at the same time the line has been extended on to the second station.  A second similar impulse from the central office will cause the relay at Station B to extend the line on to Station C, and at the same time break the circuit through the operating coil and the bell at Station B. In this way any station may be picked out by sending the proper number of impulses to operate the line relays of all the stations between the station desired and the central office, and having picked out a station it is only necessary to send out ringing current, which current is in such direction as to ring the bell but not to operate the relay magnet at that station.

In Fig. 197, a four-station line, such as is shown in Fig. 196, is illustrated, but the condition shown in this is that existing when two preliminary impulses have been sent over the line, which caused the line relays at Station A and Station B to be operated.  The bell at Station C is, therefore, the only one susceptible to ringing current from the central office.