The Engineer posted on November 10, 2006 
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Definition of the ampere:
If two long, parallel wires 1 m apart carry the same current and the force per unit length on each wire is 2x10^{7} N/m, then the current is defined to be 1 A.
Consider two long, straight, parallel wires separated by a distance a and carrying currents I_{1} and I_{2}_{ }in the same direction. We can easily determine the force on one wire due to a magnetic field set up by the other wire. Wire 2, which carries a current I_{2}, creates a magnetic field B_{2} at the position of wire 1. The direction of B_{2} is perpendicular to wire1.


The magnitude force on a length l of wire 1 is


Sincel is perpendicular to B_{2}, the magnitude of F_{1} is 


We can rewrite this in terms of the force per unit length as


The numerical value of 2x10^{7} N/m is obtained from the equation above with I_{1}= I_{2}= 1 A and a = 1 m.
The Ampère's law states that the line integral of B.ds around any closed path equals , where I is the total steady current passing through any surface bounded by closed path


Ampère's law is valid only for steady currents and is useful only for calculating the magnetic field of current configurations having a high degree of symmetry. 