Device for inducing an electrical voltage

An ignition coil for a spark ignition internal combustion engine comprising a housing, an open linear core disposed in said housing, primary and secondary inductance windings wound round said core, said core having a first part responsive to higher frequencies and a second part responsive to very low frequencies, said second part of said core having an axial length not exceeding the axial length of the first part of said core but exceeding the axial length of the primary and secondary inductance windings wound round the core, at least one of said parts of said core being a ferrite, and a body of oil in said housing in which said core and windings are immersed, said housing having a vacuum maintained therein.