A magnetic stimulator apparatus for the stimulation of neuromuscular tissue incorporates a flexible conductor 20 defining one or more multiple turn coils 24,26 and a light guide 30 which fails in the event of at least partial failure of the conductor. The light guide is preferably a fibre optic cable which extends from a light source 32, loops around at least a portion of the coil, and returns back to a light sensor. Fractures in the fibre optic cable reduce the intensity of light transmitted to the senor. When the detected light intensity falls below a threshold a warning may be triggered and/or operation of the apparatus may cease automatically. Preferably the light pipe is bonded to the flexible conductor by adhesive. The stimulator apparatus preferably has two coils connected by a bridge 28 and may be enclosed in either a flexible sheath or a rigid housing.
