2 BELL SYSTEM TECHNICAL JOURNAL 



in the general methods and arrangements employed. For the large 

 toll offices where much testing is done the measuring apparatus has 

 been installed at one or more points in an office, the installations 

 being known as transmission test boards. From these points trunks 

 radiate to repeaters, test boards, switchboards, etc., and the circuits 

 and equipment to be tested are connected to them by patching cords 

 or switchboard cords. The testing has been done by a trained force 

 of transmission testers, this force often being separate from that of 

 the test board attendants whose work has consisted chiefly in correcting 

 line faults, signaling troubles, etc., which do not require transmission 

 measurements for their location. 



In the local plant, testing has been done almost entirely with 

 portable transmission measuring apparatus in the hands of a trans- 

 mission testing force who travel from office to office, testing the cord 

 circuits, switching trunks, etc., at intervals of one or two years. 

 The measuring apparatus has been costly and its permanent installa- 

 tion in each office could not be justified. The apparatus used in 

 both local and toll testing has been described in anumber of articles.^- ^'^• 



During the last few years new methods and apparatus have been 

 developed and radical changes made that greatly improve the situa- 

 tion both from an economic and operating standpoint. The new 

 measuring instruments are much less costly than the types heretofore 

 available and are as stable in operation and as simple to use as am- 

 meters and voltmeters. In the local plant the decrease in cost and 

 complexity and improvement in operating methods has justified per- 

 manent installations in many of the larger offices while the newer forms 

 of portable apparatus are being much more extensively distributed 

 than earlier types. For the toll plant the improvement consists in 

 doing away largely with the centralized transmission testing point. 

 The regular test board force now can make transmission measure- 

 ments at their test boards and the repeater attendants can measure 

 the performance of the repeaters while working on them. The work 

 is so simple and the maintenance forces are so well trained that a 

 special transmission testing force is not required. 



This change has been brought about by the development of new 

 instrumentalities and improved circuits, chief among which are more 

 sensitive meters, the copper-oxide rectifier and the negative feedback 

 amplifier.^ Until recently, the most sensitive meters which were suit- 

 able for general use would not measure the weak power used in trans- 

 mission measuring so that amplification of this power was required. 

 Using the alloy steels now available, meters of much greater sensitivity 

 and equal ruggedness have been manufactured and transmission losses 



