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The chosen method was based on the resistivity of mercury, by measuring the resistance of a column of mercury of specified dimensions (106 cm × 1 mm 2): however, the chosen length of column was almost 3 millimetres too short, leading to a difference of 0.28% between the new practical units and the CGS units which were supposedly their basis.
As of the 2019 revision of the SI, the ampere is defined by fixing the elementary charge e to be exactly 1.602 176 634 × 10 −19 C, [6] [9] which means an ampere is an electric current equivalent to 10 19 elementary charges moving every 1.602 176 634 seconds or 6.241 509 074 × 10 18 elementary charges moving in a second.
A/m 2: A⋅m −2: U, ΔV; Δϕ; E, ξ potential difference; voltage; electromotive force: volt: V = J/C kg⋅m 2 ⋅s −3 ⋅A −1: R; Z; X electric resistance; impedance; reactance: ohm: Ω = V/A kg⋅m 2 ⋅s −3 ⋅A −2: ρ resistivity: ohm metre: Ω⋅m kg⋅m 3 ⋅s −3 ⋅A −2: P electric power: watt: W = V⋅A kg⋅m 2 ⋅s −3 ...
The emu-cgs (or "electromagnetic cgs") units are one of several systems of electromagnetic units within the centimetre–gram–second system of units; others include esu-cgs, Gaussian units, and Heaviside–Lorentz units. In these other systems, the abampere is not one of the units; the "statcoulomb per second" or statampere is used instead.
[1]: 143 For example, g/cm 3 is an SI unit of density, where cm 3 is to be interpreted as (cm) 3. Prefixes are added to unit names to produce multiples and submultiples of the original unit. All of these are integer powers of ten, and above a hundred or below a hundredth all are integer powers of a thousand.
Here, the base units are the quad, equal to 10 7 m (approximately a quadrant of the Earth's circumference), the eleventhgram, equal to 10 −11 g, and the second. These were chosen so that the corresponding electrical units of potential difference, current and resistance had a convenient magnitude. [36]: 268 [37]: 17
The electrical resistance of a uniform conductor is given in terms of resistivity by: [40] = where ℓ is the length of the conductor in SI units of meters, a is the cross-sectional area (for a round wire a = πr 2 if r is radius) in units of meters squared, and ρ is the resistivity in units of ohm·meters.
Metric units are units based on the metre, gram or second and decimal (power of ten) multiples or sub-multiples of these. According to Schadow and McDonald, [ 1 ] metric units, in general, are those units "defined 'in the spirit' of the metric system, that emerged in late 18th century France and was rapidly adopted by scientists and engineers.