we can calculate the induced e.m.f., as Lodge did in his 1889 article, considering l the closed line around the solenoid, or convenience a circumference, and S the surface having l as border. Assuming a the radius of the solenoid and r > a the radius of l , the surface crossing it is subjected to a magnetic flux π π a 2 B which is equal to circuitation l : C ( l ) = 2 π π r A ( r ) . From that stems