Current can be measured by a galvanometer, via the deflection of a magnetic needle in the magnetic field created by the
current.
The ampere, in practice often shortened to amp, (symbol: A) is a unit of electric current, or amount of electric charge per second. The
ampere is an SI base unit, and is named after André-Marie Ampère, one of the main discoverers of electromagnetism.
Definition
The ampere is a constant current which, if maintained in two straight parallel conductors of infinite length, of negligible
circular cross section, and placed 1 metre apart
in a vacuum, would produce between these conductors a force equal to 2×10–7 newton per
meter of length.[1]
The ampere is a base unit, along with the metre, the
second, and the kilogram: it is defined without reference to
the quantity of electric charge. The unit of charge, the coulomb, is defined, as a derived unit, to be the amount of charge displaced by a one ampere current in the time
of one second.
As a result, electric current is also the time rate of change or displacement of electric charge. One ampere represents the
rate of 1 coulomb of charge per second.

Explanation
Because it is a base unit, the definition of the ampere is not tied to any other electrical unit. The definition for the
ampere is equivalent to fixing a value of the permeability of vacuum to
μ0 = 4π×10−7 H/m. Prior to 1948, the so-called "international ampere" was used, defined in terms of
the electrolytic deposition rate of silver.[2] The older unit is equal to 0.999 85 A.
The ampere is most accurately realized using an watt balance, but is in practice
maintained via Ohm's Law from the units of voltage and
resistance, the volt and the ohm, since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the
Josephson junction and the quantum Hall
effect, respectively.
The unit of electric charge, the coulomb, is defined
in terms of the ampere: one coulomb is the amount of electric charge (formerly quantity of electricity) carried in a current of one ampere flowing for one second.[3] Current, then, is the rate at which charge flows through a wire or surface. One ampere of current (I)
is equal to a flow of one coulomb of charge (Q) per second of time (t):

Proposed future definition
Since a coulomb is approximately equal to 6.24150948×1018 elementary
charges, one ampere is approximately equivalent to 6.24150948×1018 elementary charges, such as electrons, moving past a boundary in one second.
As with other SI base units, there have been proposals to redefine the kilogram in such a way as to define some presently measured physical
constants to fixed values. One proposed definition of the kilogram is:
| “ |
The kilogram is the mass which would be accelerated at precisely 2×10-7
m/s2 if subjected to the per metre force between two straight parallel conductors of infinite length, of
negligible circular cross section, placed 1 metre apart in vacuum, through which flow a constant current of exactly 6 241 509
479 607 717 888 elementary charges per second. |
” |
This redefinition of the kilogram has the effect of fixing the elementary charge to
be e = 1.60217653×10-19 C and would result in a functionally equivalent
definition for the coulomb as being the sum of exactly 6 241 509 479 607 717 888 elementary
charges and the ampere as being the electrical current of exactly 6 241 509 479 607 717 888 elementary charges per second. This
is consistent with the current 2002 CODATA value for the elementary charge which is 1.60217653×10-19 ±
0.00000014×10-19 C.
CIPM recommendation
International Committee for Weights and Measures
(CIPM) Recommendation 1 (CI-2005): Preparative steps towards new definitions of the kilogram,
the ampere, the kelvin and the mole in terms of
fundamental constants
The International Committee for Weights and Measures (CIPM),
- approve in principle the preparation of new definitions and mises en pratique of the kilogram, the ampere and the kelvin so
that if the results of experimental measurements over the next few years are indeed acceptable, all having been agreed with the
various Consultative Committees and other relevant bodies, the CIPM can prepare proposals to be put to Member States of the Metre
Convention in time for possible adoption by the 24th CGPM in
2011;
- give consideration to the possibility of redefining, at the same time, the mole in terms of a fixed value of the Avogadro
constant;
- prepare a Draft Resolution that may be put to the 23rd CGPM in 2007 to alert Member States to these activities;
| This SI unit is named after André-Marie Ampère. As
for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (A). But when an SI unit is spelled out, it
should always be written in lowercase (ampere), unless it begins a sentence or is the
name "degree Celsius".
|
See also
References
- ^ Paul M. S. Monk, Physical Chemistry: Understanding our Chemical
World, John Wiley and Sons, 2004 online.
- ^ Robert B. Northrop, Introduction to Instrumentation and
Measurements, CRC Press, 1997 online
- ^ Kuzman Ražnjević, Physical Quantities and the Units of the International
System (Si), Begell House Publishers, 1995 online
External links
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