In short, [Christer] needed a high-frequency active differential oscilloscope probe, and they’re not cheap. So he built one himself. With some minor tweaking on the input damping resistors ...

They are all ways you can make oscilloscope probes that ... points out that there are different probes for different current measurement regimes. High current, medium current, and low current ...

Oscilloscope ... high voltage probes, magnetic probes, and optical probes. A coaxial test probe is a spring loaded, signal-conducting probe insulated from its shell or shielding tube by a dielectric ...