Induction coils:

Induction coils with open magnetic circuits are inefficient for transfer of power to loads. Until about 1880 the paradigm for AC power transmission from a high voltage supply to a low voltage load was a series circuit. Open-core transformers with a ratio near 1:1 were connected with their primaries in series to allow use of a high voltage for transmission while presenting a low voltage to the lamps. Various methods of adjusting the cores or bypassing magnetic flux around part of a coil were developed. The inherent flaw in this method was that turning off a single lamp affected all the others on the circuit, and many adjustable transformer designs were introduced to compensate for this problematic characteristic of the series circuit.[4] Efficient, practical designs did not appear until the 1880s,[5] but within a decade the "transformer" would be instrumental in the "War of Currents", and in seeing AC distribution systems triumph over their DC counterparts, a position in which they have remained dominant ever since.