Spin-polarized electron gun, 1970s. This is an example of the pioneering equipment used in the 1970s to probe the surface and structure of materials. It was built at the Physical Measurement Laboratory (PML) of the National Institute of Standards and Technology (NIST). This is the first spin-polarized electron gun, used to study surface magnetism. The gun generated a high-intensity unidirectional (polarised) beam of electrons to probe spin-dependent magnetic interactions at the surface of a material. The tennis ball attached to one end of the spin polarized gun is a safety feature to prevent researchers hitting their heads on the machine. Photographed in 2019, on display at NIST's Advanced Measurement Laboratory (AML), Gaithersburg, Maryland, USA.