B=ML (cose,-cose,） 4π If the wire is infinite long 01=0,02=元 B= 2πa The definition of the Ampere If two very long parallel wires one meter apart carry equal current in each is defined to be one ampere if the force per unit length on each wire is 2*10-7N/m   1 2 cos cos 4       a I B o 1= 0 , 2 =  If the wire is infinite long a I B o   2  The definition of the Ampere If two very long parallel wires one meter apart carry equal current in each is defined to be one ampere if the force per unit length on each wire is 2*10-7 N/m