402 



BELL SYSTEM TECHNICAL JOURNAL 



mission units corresponding to the ratio of any two powers Pi and 

 Po, is then the common logarithm (logarithm to the base 10) of the 

 ratio P] 'Po, divided by 0.1. This may be written N= 10 logio P\ P-i. 

 Since .V is a logarithmic function of the power ratio, any two numbers 

 of units, jVi and A'^2. corresponding respectively to two ratios, Pa/Pb 



5L3ni3rd Corrnlon Standard Cwr>mon 



Batitcry Sub-station Adjustable length of Standard Cable SattcrySjb-stat'on 



"Set -^ Set 



Fig. 1 



and Pc/Pd, may be added and the result A^ + A's, will correspond 

 to the product of the ratios, PJPbY.PclPd- 



From the above it is seen that the measure in transmission units 

 of the ratio of two amounts of power P\ and P-i is .V, where 



log 



Pi 



iV = 



log 10«' 



In other words, the transmission unit is a logarithmic measure of power 

 ratio and is numerically equal to log lO"'^. 



The reasons for the selection of this unit and the method of ap- 

 plying it, can probably be best brought out by a consideration of the 

 practise which has been followed in determining and oxprassing the 

 efficiencies of telephone circuits and apparatus in ti'rnis ot "Tuiles 

 of standard cable." 



Standard Referenck ('ik( i it 



Fig. 1 sliiiws what lias been designated the ' stamhud reference 

 circuit." It ((insists of two common liattery telephone sets of the 

 type standard in the Bell System at the time this circuit was adopted, 

 connected through repeating coils or transformers to a \ariable length 

 of "standard cable." This cable is an artificial line having a resist- 

 ance of 88 ohms and a capacity of 0.054 microfarad per loop mile 

 which is representati\e of the type of telephone cable then generally 

 used in this countrw 



