10 vernier divisions = 9 thimble divisions 
1 thimble division = 0.01 mm
10 vernier divisions =0.09 mm 
and 1 vernier division =0.09/ 10 = 0.009 mm
so the difference is the value of one thimble division – value of 1 vernier division = 0.01-0.009 mm = 0.001 mm   _{Your Comment}