$N$ divisions on the main scale of a vernier calliper coincide with $( N + 1 )$ divisions of the vernier scale. If each division of main scale is ' $a$ ' units, then the least count of the instrument is\\
(1) $a$\\
(2) $\frac { a } { N }$\\
(3) $\frac { N } { N + 1 } \times a$\\
(4) $\frac { a } { N + 1 }$