If the masses of three wires of same material are in the ratio of $1: 2: 3$ and their lengths are in the ratio of $3: 2: 1$, then electrical resistances of these wires are in the ratio

As shown in the figure, in a Wheatstone's bridge, three resistances $P, Q$ and $R$ are connected in the three arms and the fourth arm is formed by two resistances $S_1$ and $S_2$ connected in parallel. The condition for the bridge to be balanced is

The electric resistance of a certain wire of iron is $R$. If its length and radius are both doubled, then

In a meter bridge experiment the ratio of the left gap resistance to the right gap resistance is $2: 3$. The balance length from left end is