What type of X-ray tube did Rosalind Franklin use for DNA diffraction?

Rosalind Franklin used an Ehrenberg–Spear microfocus X-ray tube for DNA diffraction. The fine-focus generator had been developed at Birkbeck by Werner Ehrenberg and Walter Spear and supplied to the King’s College researchers working on DNA.

The tube was important because DNA fibers were extremely small and produced weak diffraction patterns. Its focused X-ray beam worked with a microcamera, allowing Franklin and her doctoral student Raymond Gosling to obtain clearer patterns from individual DNA fibers. Franklin also refined the equipment and developed careful methods for controlling humidity around the specimen.

This apparatus helped produce the images that revealed DNA’s helical character, including Photo 51, the famous B-form diffraction image taken by Gosling in 1952. The name can be confused with a Crookes or Coolidge tube, both historical X-ray tube designs, but those are not the specific instrument associated with Franklin’s DNA work. Later accounts also describe the device as an Ehrenberg fine-focus or microfocus tube.