34 



BELL SYSTEM TECHXIC.IL JOURNAL 



direclly across the line conductors, in which case its winding must 

 be of such high impedance that it does not materially affect the 

 efficiency of the talking circuit. Experience has indicated that with 

 the windings commonly used on relays there is some chance of suffi- 

 cient short circuited turns to materially reduce the inductance of 

 the winding. This may occur in a relay which would otherwise give 

 satisfactory operation, the short circuited turns merely reducing 





Adju5trT>eot — 





e D 66 wxwr^ turfl* 



^":i&. 



i &Ie««« Relay Contect 



Fig. 24 — Toll cord supervisory circuit 



slightly the low frequency or direct-current efficiency. For this 

 reason, the relay winding has been di\ided into two parts, on separate 

 cores, either one of which has sufificicnt inductance to safeguarti the 

 telephone transmission. 



The incoming 20-cycle signaling current may be of small value and 

 the portion through the relati\ely high impedance of the relay will 

 be still smaller so that this relay must be cxtremeh' sensiti\e. The 

 relay has small moving parts and a comparati\eIy light si)ring ten- 

 sion. These factors contribute to sensitixe operation but also permit 

 the opening of the contact on impulses other than those intended for 

 signaling. Such impulses are usually of short duration and the other 

 relays of the circuit have been designed to limit their effect to prevent 

 false signals. 



Both relay's " B " and '■ (■" are of tin- sanu- Ixpe, (icsigne;! to ojicrate 

 with a slight time lag so that other things being equal ihe\' would l)e 

 expected to operate at the same time when the circuit is closed at 

 X and y. Rela\' " B," however, receives, under the worst condition, 

 150 per cent, of its ratetl operating current, while rela\' " C" receives 

 105 per cent. This will tend to make relay "B" quicker in opera- 

 tion than relay "('," >i iliai when the battery and ground are 

 connected to the circuit, rela>- " B" will operate first and open the 

 winding of relay "C." This is therefore the normal condition of the 



