Whiting’s notes on induction-coil size

The article “Sarah Frances Whiting and the ‘photography of the invisible’ ” (Physics Today, August 2020, page 26) was fascinating. It was inspiring to learn of the important contributions she and her group made to x-ray science while using relatively modest laboratory facilities at Wellesley College.

I offer a different interpretation of Whiting’s notes on an x-ray photography experiment that was “executed with a 6 in. coil”—the induction coil used to supply high voltage to the Crookes tube that produced the x rays. In the nomenclature of the day, the maximum voltage of an induction coil was measured in inches, referring to the maximum length of air-discharge spark it could make, the most reliable way to measure high voltage at the time. A six-inch coil would generate a pulse of about 130 kV. That was a key detail to record because it related directly to the x-ray energy. The coil diameter was much less important.