522 BELL SYSTEM TECHNICAL JOURNAL 



it is possible to obtain a means of measuring a line "open" within a 

 sufficient degree of accuracy. 



A typical method of sending and receiving the timed spacing signals 

 or "opens" is shown in Fig. 12. The method of sending is shown at 

 (A), and the method of receiving at (B). To send a recall signal, the 

 operator at (A) presses the toll signal key momentarily leaving the cord 

 up, the sleeve relay therefore remaining operated. The closure of the 

 toll signal key operates relay A. Relay A operated opens the loop 

 circuit at both ends of the trunk, releasing relays B and C. Relay B 

 released closes a circuit which causes the selector to step at the rate 

 of 60 steps a minute. The release of relay C causes relay D to release 

 and provide a circuit for the selector at (B) to step at the same rate. 

 When the selector at (A) reaches the first point it locks relay A and 

 both selectors continue to step until the selector at (A) reaches the 

 seventh point, when the locking circuit of relay A opens and that 

 relay releases, closing the circuit and reoperating relays B, C, and D. 

 The reoperation of relay B energizes the release magnet of the selector 

 through the off normal contacts which cause the selector at (A) to 

 release. At (B), when the selector reaches the sixth point, relay K 

 operates and, when relay C reoperates, ground is connected through 

 the contacts of relays K, L, and M to operate relay N. Relay N 

 connects ground to the cord lamp, lighting it. Relay N when operated 

 also connects ground to relay M which locks under control of contact P. 

 When relay C reoperates ground is also connected to relay D which 

 reoperates. Battery is then connected to the release magnet and the 

 selector releases. After a time relay K, which is slow to release, also 

 releases causing relay N to release. The release of N connects ground 

 interrupted at the rate of 60 times per minute to the lamp which 

 flashes until contact P is opened by the typing key, releasing re- 

 lay M. 



To send a disconnect signal, the same operations take place at (A) 

 except that, immediately after the cord key is operated, the cord is 

 pulled down, releasing the sleeve relay, and causing the selector at 

 both ends to continue to the tenth point. At (B) when the selector 

 reaches the tenth point relay L operates and, when relay C reoperates, 

 ground is applied to operate relays M and TV which hold a steady 

 ground on the cord lamp until the cord is pulled down. 



These signals appear at all offices in a built-up connection. The 

 frequency of the machines supplying the 60 interruptions per minute 

 is accurate to within plus or minus five per cent, and the multiple con- 

 nections on the receiving selector bank take up any inequalities that 

 may exist in the speed of the machines in two different ofiftces. 



