APPLICATION OF ARTICULATION TESTING 359 



master switch on the control board the circuit conditions may be 

 switched rapidly from one to another to be compared with it. The 

 conditions changed by the master switch depend on the previous set- 



Fig. 4 — The control board and page printer. 



ting of other keys which select the individual circuit elements. By 

 these keys telephone sets of various types, which are installed on racks, 

 may be selected. Likewise, trunks with different losses and cutoff 

 frequencies are mounted on racks and are accessible through keys. 

 The magnitude of the room noise is determined by two attenuators, 

 one for each of the two circuits compared, which are adjusted before 

 the test begins and are then controlled by the master switch. Quick 

 change from one magnitude of line noise to another is managed in the 

 same way. Likewise, the setting of the automatic volume indicator 

 in the caller's control circuit is under the control of the master switch 

 so that different calling intensities can be used on successive conditions. 

 Before each list is called the operation of the switching apparatus 

 and the circuit elements are checked rapidly at the control board by 

 the application of a test tone to the transmitter terminals at one end 

 of the circuit. A volume indicator, connected across the receiver ter- 

 minals at the other end of the circuit, shows by the deflection on its 

 meter, which may be seen in Fig. 4, whether or not the power received 

 has a specified value. The same volume indicator is used to check the 

 magnitudes of the room noise and line noise. 



