What many don't understand is...
The ballast resistor protects the coil from overcurrent...
Its resistance increases as it gets hotter..
Traditional coils had thin internal wire windings and could not handle higher currents.

Some later coils are designed to handle higher currents, also some do not require a ballast resistor U need to check the maufacturer for their recommendations. Now the traditional oil-filled coil is being replaced with the EL transformer type design, which is more compact..

Just my $0.02...