TELEPHONE SYSTEM OF THE UNITED STATES 21 



provement in the service. Some of the more important of these 

 changes are as follows : 



1. Automatic ringing which continues automatically at regular inter- 



vals until the subscriber answers. 



2. Ringing tone, very much reduced in volume, to the calling sub- 



scriber automatically advising him when ringing is in progress. 

 ,1. The audible busy signal, a tone placed on the calling line when 

 the called line is busy. 



An important recent change in manual central ofhce equipment 

 relates to the trunking methods employed in completing a connection 

 from one central office to another. In most of the larger cities the 

 so-called "straightforward" method is used. With this operating 

 plan the number that is desired in the distant office is passed by the 

 originating or "A" operator to the completing or "B" operator over 

 the trunk that is used for completing the trunk connection. This is 

 in contrast to the "call circuit method" where all orders between 

 operators are passed over a separate wire known as a call circuit. 

 The principal equipment changes at the "B" positions have to do 

 with the different circuit plans for connecting the trunk operator's 

 telephone set to the trunk. This is done either by means of a key, 

 by means of plugging the trunk into a listening jack or automatically 

 by means of suitable relays. At the "A" end the principal change is 

 the arrangement for testing whether or not an outgoing trunk is 

 busy. This is done either by means of a lamp indicating a free trunk 

 or by a lamp or tone indicating that a part of a trunk group is free. 



Dial Equipment 



Dial equipment of two types known as the step-by-step system 

 and the panel system respectively are used in about equal amounts 

 in the Bell System. The change from manual to dial operation pre- 

 sented a very large problem from an engineering, a manufacturing, 

 an installing and an economic standpoint. At first the dial installa- 

 tions were to care largely for growth but they have been followed by 

 installations for the replacement of existing manual equipment where, 

 all factors considered, this was clearly justified. In this way an or- 

 derly program has been developed. Figure 15 indicates the total 

 number of stations on a dial and on a manual basis for each year 

 since 1921 and the expected program up to 1933. Under the present 

 contemplated dial program it is estimated that the areas employing 

 step-by-step equipment will be on a complete dial basis by 1937 and 

 that all areas employing panel equipment will be substantially com- 

 pleted by 1942. 



