Sallie Pero Mead was first hired at AT&T in 1915 as a “computer”—a human calculator—shortly after completing her master’s degree in mathematics at Columbia University. Before long she started working on the company’s transmission engineering team as both a mathematician and an electrical engineer. She and her team developed and tested hollow metal tubes used as waveguides: structures that confine and direct electromagnetic waves. In 1933 they discovered a new way that hyperfrequency waves could propagate down these tubes, and this made radar technology possible—just in time for use in World War II.