Define the tesla.
A large horseshoe magnet produces a uniform magnetic field of flux density 
between its poles. Outside the region of the poles, the flux density is zero. The magnet is placed on a top-pan balance and a stiff wire XY is situated between its poles, as shown in the figure below.
The wire XY is horizontal and normal to the magnetic field. The length of wire between the poles is 
. A direct current of magnitude 
is passed through the wire in the direction from X to Y. The reading on the top-pan balance increases by 
.
State and explain the polarity of the pole P of the magnet.
Calculate the flux density between the poles.
The direct current in (b) is now replaced by a very low frequency sinusoidal current of r.m.s. value . Calculate the variation in the reading of the top-pan balance.
producing a force per unit length of 
.
.
.
