Novel sensors have been developed to measure the time resolved position and thickness of the conducting zone of detonation waves in columns of explosive. The sensors employ the phenomenon of capacitive coupling to distinguish between the reaction zone, unreacted explosive and reaction products based on the change in conductivity. An electrode is driven with a square-wave alternating voltage, which induces a stronger displacement current in the conducting reaction zone than elsewhere. The size of the coupling effect is then inferred by measuring the relaxation time of the circuit. Unlike some other electromagnetic sensors, these are not embedded within the material, but instead are designed to be positioned on the outer surface. This paper describes the design of the sensors and an initial test. A fibre-coupled laser flyer system has been built to perform initiation experiments.