A dimensionless unit used in telecommunications to express the ratio between two measurements of power. Symbol, Np. The neper is the natural logarithm of the power ratio.

nepers equal one half log to the base e of the fraction P sub one over P sub two

If the input and output impedances are the same, then for two voltages or two currents:

nepers equal log to the base e of the fraction V sub one over V sub two   and   nepers equal log to the base e of the fraction I sub one over I sub two

Like the decibel, which performs the same functions, the neper came into use in the middle 1920's, replacing the mile of standard cable (m.s.c.). The decibel, however, became popular in Britain and America and is based on log to the base ten, while the neper was used in continental Europe and the base of its logarithm was e. One neper = 8.686 decibels.

The neper is named for John Napier (1550 – 1617), the Scottish mathematician who invented logarithms to the base e.

The neper is not currently an SI unit. A proposal to include the neper as an SI unit was recommended by the CCU, approved by the CIPM, but withdrawn at the 21st CGPM (October 1999). Regarding its possible adoption, see footnote 3 in decibel.

Consultative Committee for Units (CCU).
Report of the 14th meeting (19-20 April 2001) to the International Committee for Weights and Measures.

See Section 4, Proposal to Adopt the Neper as an SI Unit, page 58, and Recommendation 1 (CI-2001): The neper and the bel. page 70.

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