Voltage drop across the injector plus current clamp around the power wire.
He went back to the PDF. He used the search function: 'Reduced Oscillations, Single Cylinder' . The document was hundreds of pages long, a dense forest of technical data. But Arthur knew that the answer to a riddle is rarely in the question; it's in the footnotes.
The PDF showed a healthy waveform: a clean vertical spike (the firing line), a steep drop, and then a gentle, oscillating hump (the spark burn line) until the current was cut. It looked like a mountain peak with a gentle slope behind it.