When a long iron-core solenoid is connected across a 6-V battery, the current rises to 0.63 of its maximum value after a time of 0.75 s. The experiment is then repeated with the iron core removed. Now the time required to reach 0.63 of the maximum is 0.002 5 s. Calculate 

(a) The relative permeability of the iron and 

(b) L for the aircore solenoid if the maximum current is 0.5 A.