With a resistance '$X$' connected in series with a galvanometer of resistance $100\ \Omega$, it acts as a voltmeter of range 0 - 15 V. To double the range, a resistance of $1500\ \Omega$ is to be connected in series with '$X$'. The value of '$X$' in $\Omega$ is

If the potential difference between points B and D is zero in the given circuit, the value of x is $\dfrac{1}{n}\ \Omega$. The value n is

A potentiometer wire is 4m long and potential difference of 3V is maintained between the ends. The e.m.f. of the cell which balances against a length of 100 cm of the potentiometer wire is

A metre bridge experiment, the resistance in the left gap is $20\ \Omega$ and in the right gap is $60\ \Omega$. The bridge is balanced. The distance of the null point from the centre of the wire is