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.

J. S.
J. M.
Physics Today