Page:Electricity (1912) Kapp.djvu/154

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

$B={\frac {2\pi J}{R}}$

By means of this formula we may now define unit current. If $J=1$ and $R=1$, then $B=2\pi$. 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 $F={\frac {2\pi Jm}{R}}$ dynes on the magnetic mass $m$ 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 $m$ acts on the wire. The induction due to $m$ in the space occupied by the wire is $B={\frac {M}{R^{2}}}$, and by combining the equations for $F$ and $B$ we find $F=2\pi RJB$ dynes.

This is the force experienced by a wire of