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BELL SYSTEM TECHNICAL JOURNAL 



electrical power without distortion. In connection with these ampli- 

 fiers a sharp distinction should be made between their gain rating, 

 or amplification, and their overload or power rating. Gain measures 

 the power amplification which can be obtained provided the input 

 is small enough so that the equipment at the output end is not over- 

 loaded. Overload or power rating refers to the maximum power 

 which can be supplied by the amplifier without causing distortion 

 of the currents being amplified. Although the power rating of power 

 equipment is usually determined by the heat which can be dissipated, 

 a marked distortion of wave form takes place when the iron in any 

 of the apparatus is worked beyond the straight line portion of the 

 magnetization curve. In the case of amplifiers, the maximum power 

 obtainable is limited by the power output at which distortion occurs 

 rather than by the heat which can be dissipated. 



Fig. 3 shows a chart for the characteristics of the complete system, 



"240 480 960 1920 3840 



FREQUENCY 



Fig. 3 — Complete Public Address System with Carbon Transmitter. 



including the carbon transmitter, the speech input and power ampli- 

 fiers and the receiver projector unit. 



In connection with the requirements for equal amplification of all 

 frequencies, it is interesting to note that a system, which does not 

 fail to reproduce equally all frequencies in the speech range by more 

 than a ratio of 10 to 1 in reproduced power, is indistinguishable from 

 a perfect system or from the speaker, himself. It seems probable 

 that this effect is, in some way, connected with variations in the fre- 

 quency sensitiveness curve of normal ears. Normal ears show a 

 sensitiveness variation with frequency as great as 10 to 1 and the fre- 

 quencies of maximum sensitiveness vary materially from one in- 

 dividual to another. 7 



7 Frequency Sensitiveness of Normal Ears, by H. Fletcher and R. L. Wegel, 

 Physical Review, July, 1922. 



