Various types of pulse generators are known, and one particularly suitable construction utilizes a Wiegand wire. Wiegand wires, now well known, have the characteristic that, upon being subjected to a variable magnetic field, the transition of magnetization is abrupt and independent of the rate of change of a magnetic field to which the wire is exposed. Thus, the amplitude of the output pulse will not vary with rate of change of flux.
A contactless pulse source utilizing a Wiegand wire to trigger ignition pulses has been described in "Electronics", 1977, page 85. In the structure there disclosed, a Wiegand wire is embedded in, or forms the core of, a pick-up coil. Two permanent magnets are located in the magnetic field of the coil, and spaced therefrom. The permanent magnets are of opposite polarity. A rotating magnetic shield forming a diaphragm or mask rotates between the magnets and the coil, such that the magnetic field through the Wiegand wire changes its direction each time an opening in the mass or diaphragm passes the pick-up coil. This pulse source requires two permanent magnets and associated mounting structure to position the two permanent magnets relative to the coil.