4< v 6 



NATURE 



[October 12, 191 1 



THE AUTOMATIC TELEPHONE EXCHANGE. 



AT an ordinary or manual telephone exchange, as 

 is generally known, the subscribers' lines ter- 

 minate on "jacks," and are put through to each other 

 by means of " plugs " and flexible conductors. The 

 jacks, mounted on a suitable surface, form a "switch- 

 board," and it is the business of the exchange operator 

 to make the necessary connections and to sever them 

 at the proper time, to answer calling subscribers and 

 to ring up wanted ones. Large modern exchanges 

 are worked on the common-battery plan ; that is, no 

 batteries whatever are placed at the subscribers' 

 offices, but a large single battery is installed at the 

 exchange, and this supplies all the current for speak- 

 ing and signalling. The switchboard is of the 

 "multiple" type. All the subscribers are brought to 

 each division or section of the complete board ; that 

 is. all the subscribers' jacks in the entire exchange 

 are repeated or multipled at each section, the latter 

 forming a kind of unit of area representing the 

 maximum reach of an operator. Every subscriber is 

 thus within the reach of every operator. A simple 



" busy " test is 

 arranged, so that 

 before making a 

 connection it can 

 instantly be seen 

 whether the re- 

 quired subscriber 

 is free or already 

 engaged at some 

 other section. This 

 grouping of the 

 jacks, or "mul- 

 tiple " proper, is 

 used only for ring- 

 ing up and con- 

 necting to wanted 

 subscribers. I n 

 addition, each 

 operator has a 

 certain number of 

 s u b s cribers 

 brought to other — 

 "an s w ering " — 

 jacks, placed in 

 her immediate 

 vicinity, and it is 

 on these that calls 

 from subscribers 

 are received. At 

 the subscriber's 

 end everything is arranged with a view to simplicity. 

 The signalling is automatic ; that is, the subscriber 

 has simplv to lift his receiver and the exchange is called 

 automatically, and when the conversation is finished 

 the replacing of the receiver upon its hook advises the 

 operator that the connection is to be severed. 



For automatic exchange working we have to substi- 

 tute mechanism for the exchange operator, and the 

 place of the large common switchboard is taken by a 

 number of small individual switches, one of which is 

 allotted to each subscriber and is entirely under his 

 control. The plan which we are about to outline is 

 the Strowger system, as applied to a small exchange 

 of less than a hundred subscribers and operated on 

 the common-battery plan. Other excellent systems 

 are in use, but it is thought that some degree of 

 detail of one system will be more useful than the 

 statement of the main principles of several. 



Considering the subscriber's end first. In addition 

 to the usual speaking apparatus, transmitter, receiver, 

 &c., we have the selecting and calling mechanism, as 

 the subscriber has to "get through" to his corre- 



XO. 2l8g, VOL. 87] 



spondent by his own unaided efforts. Fig. i illus- 

 trates a "table set." In the central portion the figures 

 1 to o are arranged in a circle, and over them is a 

 movable disc, perforated near the edge so that the 

 figures, which are fixed, are visible through the holes. 

 Suppose subscriber No. 58 is required. The receiver 

 is first removed from its hook. A finger is then in- 

 serted in the hole through which the figure 5 shows, 

 and the disc turned in a clockwise direction until a 

 stop bars the way. This turning of the disc winds 

 up an inner spring. The finger is then withdrawn 

 and the spring carries the disc back again. In its 

 return journey it causes one of the lines of the loop 

 to be earthed (momentarily) five times, and at the 



-Details of Subscriber's Switch a 



liange 



end of this series the other line is earthed once. The! 

 finger is then inserted in the hole over the 8 and the 

 disc brought round again — again winding up the 

 spring;. lis release and return to normal earths the 

 first line eight times and then the second line once, as , 

 before. As the exchange battery is permanently on 

 the lines, this earthing causes impulses of current to be 

 sent over them by way of certain apparatus, and the 

 calling subscriber is now through to No. 58 line. 

 When any subscriber's receiver is on its hook, a 

 magneto bell, in series with a condenser, is across the 

 two lines. The calling subscriber now depresses a 

 push-button, which action again earths the first wire 

 of his own loop and actuates a relay at the exchange, 

 which applies the generator to the required line. 



