132 BELL SYSTEM TECHNICAL JOURNAL 



3. Sensitivity 



The application of a 1000-cycle potential of 1.228 volts r-m-s (4 

 decibels above 1 milliwatt in 600 ohms) to the instrument in series 

 with the proper external resistance causes a deflection to the vu or 

 100 mark. The instrument therefore has sufficient sensitivity to be 

 read at its normal point (0 vu or 100) on a volume level of + 4 vu.'^ 



4. Harmonic Distortion 



The harmonic distortion introduced in a 600-ohm circuit by bridging 

 the volume indicator across it is less than that equivalent to 0.2 per 

 cent (r-m-s). 



5. Overload 



The instrument is capable of withstanding, without injury or effect 

 on calibration, peaks of 10 times the voltage equivalent to a deflection 

 to the vu or 100 mark for 0.5 second and a continuous overload of 

 5 times the same voltage. 



6. Color of Scale 



The color of the scale card, expressed according to the Munsell 



9.18 ^ 

 system of color identification, is 2.93Y j-rr . 



7. Presence of Magnetic Material 



The presence of magnetic material near the movements of the 

 instruments as now made will affect their calibrations and dynamic 

 characteristics. This is because it has been necessary to employ more 

 powerful magnets than usually required for such instruments to obtain 

 the desired sensitivity and dynamic characteristics, and any diversion 

 of flux to nearby magnetic objects effectively weakens the useful mag- 

 netic field beyond the point where these characteristics can be met. 

 The instruments should not, therefore, be mounted on steel panels. 

 (The effect is only slight if they are mounted on 1/16 inch panels with 

 the mounting hole cut away as far as possible without extending 

 beyond the cover of the meter.) 



^ There should be no confusion because the instrument deflects to a scale marking 

 of vu when a level of -f- 4 vu is applied to it. As in previous volume indicators, 

 the vu point on the vu scale is merely an arbitrary point at which it is intended 

 nominally to read the instrument, and the rest of the vu scale represents deviations 

 from the vu point. The volume level is read, not from the scale, but from the 

 indications on the associated sensitivity control when the latter is so set as to give 

 a scale deflection to the vu mark. If a deflection other than vu is obtained, the 

 volume level may be corrected by the deviation from vu shown on the instrument 

 scale. In the present art, it is difficult to make an instrument of the desired 

 characteristics having a sensitivity greater than that indicated. 



^ Munsell Book of Color, Munsell Color Company, Baltimore, Maryland, 1929. 



