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why coils in a resistance box are made up of doubled up insulated coils?

This is done to minimize the inductance of coils.The wire is doubled back on itself.As a result ,there are equal and opposite currents in each section of the coil.Therefore ,the coil has no net magnetic field and no net induced e.m.f.Such coils are called non - inductive coils and have the following advantages :
(i) The current attains the final value quickly at the time of make or break.
(ii) When used with a.c.,the current through such coils is not frequently - dependent.


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