154 



BELL SYSTEM TECHNICAL JOURNAL 



stantially the same as unbiased reversals at about 11 d.p.s.). The 

 bias component is simply the bias measured with / signals, the fortui- 

 tous component is taken as the difference between the total distortion 

 and the bias of / signals, and the characteristic component is obtained 

 by averaging the results for the 6 selected characters and then selecting 

 the result which shows the maximum departure from the average. 



These sets also furnish a convenient means for the measurement of 

 mean-square values of distortion. The current impulses flowing 

 through the total distortion meter are proportional to the distortion 

 and it is practicable to insert in series a specially arranged meter 

 which is calibrated to indicate the mean-square values of the dis- 

 tortion. The circuit used is shown by Fig. 6, the heavy lines showing 



LOOP 

 RELAY C 



TOTAL 



DISTORTION :: 



(PEAK- indicating) 



C2 



copper-oxide 

 rectifiers 



MEAN- SQUARE - 

 DISTORTION 



Fig. 6 — Circuit of start-stop set for measurement of mean-square distortion. 



the meter circuit which is inserted in series. It contains an integrating 

 meter whose characteristic is modified by a full-wave copper-oxide 

 rectifier, a large condenser for increasing the damping and resistances 

 for adjusting the amplitude and response characteristics. Measure- 

 ments of mean-square distortion have been found of value in con- 

 nection with producing transmission ratings of telegraph circuits, 

 these ratings being based on the assumption that the mean-square 

 values of distortion of component parts of a circuit may be added 

 directly to predict the total mean-square distortion.^ 



