Geoffrey Opat is distinguished for his skill in developing clever experimental techniques for obtaining answers to important questions in physics. His first widely known exhibition of this skill was the design of a practical realisation of Bernstein's concept for demonstrating the change in sign associated with the rotation of a spinor through 360°. More recently he has attracted international attention through his experimental investigation of the patch effect at low temperatures, of importance in the design of the proposed experiment on detection of the gravitational acceleration of the antiprotons. He also gained widespread recognition through his participation in the observation of Aharanov­Casher effect.