To convert a galvanometer into a voltmeter one should connect a

A galvanometer of 50 ohm resistance has 25 divisions. A current of 4 $$ \times $$ 10^$$-$$4 ampere gives a deflection of one division. To convert this galvanometer into a voltmeter having a range of 25 volts, it should be connected with a resistance of

A charged particle moves through a magnetic field in a firection perpendicular to it. Then the

A long solenoid carrying a current producess a magnetic field B along its axis. If the current is doubled and the number of turns per cm is halved, the new value of the magnetic field is