1. In a molecular beam tube apparatus including source means for projecting molecular particles, a first state selector for forming said projected particles into at least one molecular beam, an r.f. transition section disposed downstream from said first state selector for effecting resonance of the beam particles, means for producing a polarizing magnet field in the r.f. transition section, a second state selector disposed downstream from said r.f. transition section for deflecting selected molecular particles in the beam, and a detector for receiving molecular particles from said second state selector to indicate when molecular resonance occurs in said r.f. transition section; the improvement wherein said means for producing a polarizing field comprises: an elongated structure constructed of magnetically permeable material and having a longitudinal axis aligned with the path of said molecular beam, said structure being configured to at least partially surround said beam in said r.f. transition section; conducting means for conducting a first current within said elongated structure to produce a homogenous magnetic polarizing field in the path of said molecular beam, and conducting a second current for degaussing said elongated structure; source means coupled to said conducting means for providing said second current, said source means including: current generator means for providing an output current which decays with a predetermined time constant; oscillator means for providing an output signal at a predetermined low frequency; and polarity reversing means responsive to said oscillator means for coupling the output of said current generator means to said conducting means, and reversing the polarity of the output current from said current generator means in synchronism with the output signal from said oscillator means.

9. In a molecular resonance apparatus including a molecular interaction region for effecting resonance of molecules by electromagnetic field excitation, and means for magnetically shielding said molecular interaction region, an apparatus for degaussing said magnetic shielding means comprising: current conducting means disposed adjacent to said magnetic shielding means for producing magnetic field currents in said shielding means; source means for providing an alternating, decaying current to said current conducting means, thereby to produce an alternating, decaying magnetic field in said shielding means, said source means including: current generator means for providing an output current which decays exponentially with a predetermined time constant; oscillator means for providing a square wave output signal at a predetermined low frequency; polarity reversing switch means for coupling the output of said current generator means to said current conducting means, said switch means having control input means coupled to the output of said oscillator means for reversing the polarity of the exponentially-decaying current output from said current generator in synchronism with said square wave signal.