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The ESU and the EMU were developed for scientific work in electrostatics and electromagnetics, respectively.
The sizes of the units in these systems are not convenient; e.g., one abfarad equals 1 X 10′ farads, one abvolt equals 1 X 10″8 volt, one statohm equals 8.987 X 10″ ohms, etc. When using the ESU and EMU, one must be very familiar with the conversion of units between these systems.

The MKS electromagnetic system evolved or was developed to give a single order with units of practical sizes that can be used conveniently in all areas of research and engineering involving electrical,
magnetic, and electromagnetic phenomena. The rationalized MKS (RMKS) system incorporates definitions of permeability and permittivity, which result in the factors l/(2ir) and l/(4ir) appearing in the equations describing electric and magnetic phenomena having cylindrical and spherical symmetry, respectively.

(This is discussed briefly in par. 4-7 (Ref. 6).)
Table 4-5 is included to assist in the identification of equivalent units in the SI, MKS, ESU, and EMU. Conversion factors for many of the groups in Table 4-5 are given in Tables 5-1 and 5-2. In those cases for which conversion factors are not provided, the methods in par. 4-4 for the conversion of derived quantities should be used. Par. 4-7, which is concerned with equations and formulas, is relevant to the transformation of
electromagnetic units, particularly when going from EMU or ESU to the RMKS or SI.

During the transition to much broader use of the SI; scientists, engineers, and technicians frequently may be faced with the necessity of using equations and formulas developed for use in any one of several systems of units other than the SI. In general, this will involve one of two situations. for info https://scientificnotationconverter.info/