Define the tesla.
A long solenoid has an area of cross-section of 
, as shown in the figure below.
A coil C consisting of 160 turns of insulated wire is wound tightly around the centre of the solenoid. The magnetic flux density 
at the centre of the solenoid is given by the expression
where 
is the current in the solenoid, 
is a constant equal to 
and 
is the permeability of free space. Calculate, for a current of 
in the solenoid
the magnetic flux density at the centre of the solenoid,
the flux linkage in the coil C.
producing a force per unit length of 
.
.
.
