The force acts at right angles to the plane of the circular loop. A unit pole will therefore be drawn through the loop with the force
By means of this formula we may now define unit current. If and , then . Now imagine a wire bent into a circle of 1 cm. radius. If unit pole in the centre of this circle is drawn through it with a force of 6.28 dynes, then there is unit current in the wire. This so-called electromagnetic unit of current strength is too large for practical work, and for this reason a unit ten times smaller is adopted. This is called the ampere.
The coil exerts a force dynes on the magnetic mass placed in the centre of its plane, but since action and reaction must always be equal, this is also the force with which the mass acts on the wire. The induction due to in the space occupied by the wire is , and by combining the equations for and we find dynes.
This is the force experienced by a wire of
