The magnetic field produced by a coil is proportional to the current I in the coil and the number of turns n and inversely proportional to the radius R of the coil.
http://en.wikipedia.org/wiki/Helmholtz_coil
Google magnetic field in a coil.
http://hyperphysics.phy-astr.gsu.edu/Hbase/magnetic/curloo.html
http://en.wikipedia.org/wiki/Helmholtz_coil
Google magnetic field in a coil.
http://hyperphysics.phy-astr.gsu.edu/Hbase/magnetic/curloo.html