The invention is directed to improvement of a write element of a thin film magnetic head. The cathode and the anode each have a reactant feed and a reactant discharge, a humidifying device and sensors being provided at least at one of the reactant feeds. The thin-walled cylindrical member has: a first crimped portion, in which an axially intermediate portion is reduced in diameter in a radial direction, and is crimped to the shaft member ; and a second crimped portion, in which an end surface is folded radially inward, and is crimped to the shaft member. By considering a plurality of wafers as a unit, a single measurement representing a combination of individual wafer responses is obtained.