A monitoring apparatus configured for monitoring a carbon brush of a brush holder assembly of an electrical machine. The monitoring apparatus may include a flexible sensor and a signal processing circuit for processing a signal received from the flexible sensor. The flexible sensor may have an electrical resistance that varies based on a radius of curvature of the flexible sensor, wherein the radius of curvature of the flexible sensor may be associated with a deflection of a spring providing a force to engage the carbon brush with a rotating component of the electrical machine. The signal processing circuit may be coupled to the sensor and may be configured to determine a measure of a wear state of the carbon brush using information about the variable resistance of the flexible sensor.