Solid $NaCl$ is a bad conductor of electricity because

The two lenses of an achromatic doublet should have:

In the circuit shown in the adjoining figure,the reading of the ammeter is .............. $A$.

The half-life period of a first-order chemical reaction is $6.93 \, \text{min}$. The time required for the completion of $99 \%$ of the chemical reaction will be .......... $\text{min}$ $(\log \, 2 = 0.301)$.

Two uniform strings $A$ and $B$ made of steel are made to vibrate under the same tension. If the first overtone of $A$ is equal to the second overtone of $B$ and if the radius of $A$ is twice that of $B,$ the ratio of the lengths of the strings is

Two thick wires and two thin wires,all of the same material and same length,form a square in three different ways $P$,$Q$,and $R$ as shown in the figure. With the current connection shown,the magnetic field at the centre of the square is zero in which cases?