Diagram showing attractive force on other wire.

Obtaining an expression for force.

Definition of one ampere.

As shown in Figure, the direction of force on conductor b is attractive
[Alternatively:

→

B

at a point on wire 2 , is along −

^

k

∴

→

F

, on wire 2 , due to the

→

B

, is along −

^

i

, i.e. towards wire1. Hence the force is attractive.
Magnetic field, due to current in conductor a ,
B1=‌

µ0I1

2πd

The magnitude of force on a length L of conductor b ,
‌F2=I2LB1
‌F2=‌

µ0I1I2L

2πd

One ampere is that steady current which, when maintained in each of the two very long, straight, parallel conductors, placed one meter apart in vacuum, would produce on each of these conductors a force equal to 2×10−7 newton per meter of theirlength.