NEW TRANSMISSION MEASURING SYSTEMS 



13 



are equipped with them. Two arrangements are available for test 

 board use. One employs a conventional type of meter mounted in 

 the keyshelf or on a panel and the other employs the projection type 

 of meter which is illustrated in Fig. 9.^ The method of operation is 

 simple. When a transmission test is to be made the tester listens 

 until he hears the tone caused by testing power coming over the circuit 

 from the distant generator, then connects the circuit to a jack in which 

 the measuring set input terminates. The meter indicates immediately 

 the net loss of the circuit at the testing frequency. With the projec- 

 tion meter arrangement the lamp in the projector is turned on automat- 

 ically when a connection is made to the test jack. 



The new amplifier-rectifier will measure transmission losses and 



Fig. 10 — Noise measuring set. 



gains and also transmission level. For the latter type of measurement 

 the input impedance of the amplifier is raised to a high value so that 

 it is, in effect, a voltmeter. This change in impedance can be made 

 from a remote point, a relay for making the change being a part of 

 the amplifier. 



In addition to the measurement of transmission losses, gains and 

 levels on telephone circuits, it is also necessary at times to make 

 measurements of noise on the circuit. This noise may be caused by 

 currents induced by power systems or from power plants in the 

 telephone offices or it may be in the form of crosstalk, sometimes un- 

 intelligible, from other telephone circuits. Noise measurements are 

 now made with meter indicating devices, the latest type of self- 

 contained portable noise measuring set being shown in Fig. 10.^ 

 Where enough noise measurements are made to justify a permanent 

 installation, an arrangement similar to that described for transmission 



