While modest solenoid currents can be maintained steady and measured by DMM-as-ammeter, pulsed currents necessarily come with a waveform, and users will need a way to know the instantaneous current i(t), not just its peak value.  With our Pulsed Current Source, we provide two routes to this knowledge.

A first method comes from a ‘current shunt’ – the solenoid current i(t) passes through a shunt of known resistance, and the resulting voltage drop is detected differentially, and made available to the user, conveniently ground-referenced.  Combined with a known solenoid constant (about 11 mT/A) such i(t) information allows reconstruction of the B(t) waveform the solenoid delivers.

An entirely independent method comes from Faraday’s Law of Induction – a pick-up coil mounted inside the solenoid (and outside the Faraday-rotation optical sample) delivers a real-time emf proportional to the rate-of-change of the field during the pulse.  Knowledge of turns-count and turns-area allows the ‘scope-recorded emf to produce quantitative dB/dt information in Tesla/second; then after-the-fact integration allows another and independent way to recover the time history of B(t) during the pulse.

But of course the glamor experiment is to see what happened to the light beam’s polarization during the brief (<10 ms) pulse of current and field.  TeachSpin’s Faraday-Rotation optical bench already includes the analyzer, and the time-nimble photodetector, needed for recording the time history of the intensity of light passing through the Polaroid analyzer.  Typical experiments include pre-setting the analyzer’s pass-direction to 90°, or to 0°, relative to the input light’s polarization; but the best real-time detection of Faraday rotation comes from setting that angle to 45°, and then recognizing what happens when Faraday rotation causes temporary excursions relative to that value.

Our Manual includes a chapter on the time-reversal properties of electromagnetism, and the use of Faraday Rotation to evade the otherwise strict reciprocity conditions of optics, plus the use of 45° Faraday rotators to create ‘optical diodes’ or ‘optical isolators’ for light.