382 



BELL SYSTEM TECHNICAL JOURNAL 



plete testing equipment is available, by means of which, a large 

 volume of transmission testing in central offices and private branch 

 exchanges can l)e done in the most convenient manner. Tests using 

 these instruments can be made as readily on the transmission circuits 

 in machine switching offices of both the step by step and panel types as 

 in manual offices. The ear balance set of Fig. 24 requires no ex- 

 ternal source of direct current power and only a three dry cell battery 

 is required for operating the oscillator. It is, therefore, used to the 

 best advantage in testing private branch exchange switchboards and 

 magneto .switchboards where the power necessary to operate visual 

 types of sets is not readily available. 



The visual type of set of Fig. 25 is pariicularK- suited lor testing 

 in the larger common battery central offices since it i)ermits measure- 

 ments to be made more quickly and acrnrate1\- than in the caso of the 



Fig. 26 



ear balance sets. Tiiese larger oftices also ha\e readih' axailable 

 the 24-\<)It batteries re(|uired to operate the \isual reading sets. 

 P'ig. 26 shows a picture of one of the latest types of portable \isual 

 reading measuring sets, set up reach" for operation at a central otlue 

 switchboard position. 



