A long solenoid of radius 1mm has 100 turns per mm.  If 1 A current flows in the solenoid, the magnetic filed strength at the centre of the solenoid is 

Magnetic fi eld due to a solenoid at its centre is $\mathrm{B}={\mu }_0\mathrm{ni}$ , where ‘n’ is the no. of turns per unit length and ‘i’ is the strength of the current.
Here, $\mathrm{n}=\frac{100}{{10}^{-3}}={10}^5\text{ turns }/\text{ metre }$
i = 1 A
So, $B=4\pi \times {10}^{-7}\times {10}^5$
$=4\pi \times {10}^{-2}=12.56\times {10}^{-2}\mathrm{T}$