APPLICATION OF ARTICULATION TESTING 363 



number are operated an electromagnetically stepped selector switch 

 is actuated. If less than this number of relays are operated another 

 selector switch is actuated. If exactly the right number of relays are 

 operated neither selector switch is actuated. These selector switches 

 control the moving parts of a differential switch, which consists essen- 

 tially of a rotating set of contact points and a rotating contact arm 

 which may move over these contact points, the direction of rotation 

 being the same for both. Each contact point is connected to one of 

 the signal lamps in the calling booth, the lamp circuit being com- 

 pleted through the contact arm to a battery. At the beginning of a 

 list the contact arm rests on the contact which illuminates the center 

 lamp. If a test sentence is called at too high an intensity the contact 

 arm moves over a number of contact points governed by the number 

 of relays operated in excess of the desired number, illuminating the 

 coordinated lamp. If the following sentence is called at the proper 

 intensity neither selector moves and, therefore, the same lamp remains 

 illuminated. If, on the next sentence, the calling intensity is low, 

 less than the specified number of relays are operated; consequently, 

 the other selector switch moves the contact points the required num- 

 ber of steps while the contact arm remains stationary. This, in effect, 

 moves the contact arm backward to the contact point corresponding 

 to the algebraic sum of the deviations from the desired value up to 

 this point in the list and this is shown in the calling booth by a move- 

 ment of the light to the left. In a similar way the succeeding devia- 

 tions from the average cause the two parts of the differential switch 

 to move in such relation to each other that the signal light indicates 

 the cumulative departure from the desired average value. 



Reversing the Direction of Transmission 

 The use of the caller's control circuit and an automatic switching 

 system permits reversal of the direction of transmission over the cir- 

 cuits tested without loss of time. In addition to soundproof booths 

 for the caller and the observers, two others are used. These two 

 booths, which are in all respects identical, are referred to as sidetone 

 pickup booths. Each contains an artificial mouth, a number of trans- 

 mitters which may be connected to the circuit tested, and a loud- 

 speaker which reproduces room noise. These booths are shown sche- 

 matically in Fig. 5 and a photograph of the interior of one is shown in 

 Fig. 7. 



The timing commutator, which governs the other automatic devices 

 used, controls switches which make the following connections (see 

 Fig. 5) : The artificial mouth in Booth I is connected to the caller's 



