Important contributions to radio astronomy, particularly to the subject of the discrete sources ("radio stars"). Obtained first observations and later confirmed that the discrete sources are of more than stellar dimensions. Gave first evidence that the discrete sources fall into two classes, now recognised as galactic and extra-galactic respectively. Developed various new techniques including a form of pencil-beam radio telescope ("Mills Cross") permitting very great apertures, e.g. 1500 feet in current model, and consequent high resolving power. Also worked during war years on radar development and subsequently applied radar techniques to make million-volt X-ray generator.