Decibel - The name for the transmission unit

In 1923 the “mile of standard cable” was replaced in the Bell System by a new unit for expressing telephone transmission efficiencies and levels. At that time, the generic term “transmission unit” was taken to designate this new unit, since it was considered desirable to defer the adoption of a more distinctive name until this unit had been given further consideration by others who would have use for a unit of this type. This new unit is defined by the statement that two amounts of power differ by one transmission unit when they are in the ratio of 1·¹, and any two amounts of power differ by N transmission units when they are in the ratio of 10^N(.1). In accordance with this, the number of transmission units corresponding to the ratio of any two powers is ten times the common logarithm of that ratio. [Originally reported in the Bell System Tech. Journal, January, 1929].