An electronic device, such as a watch, has a crown assembly having a shaft and a user-rotatable crown. The user-rotatable crown may include a conductive cap that is mechanically and electrically coupled to the shaft and functions as an electrode. The conductive cap may be coupled to the shaft using solder or another conductive attachment mechanism. The shaft may electrically couple the conductive cap to a processing unit of the electronic device. One or more additional electrodes may be positioned on the exterior surface of the electronic device. The conductive cap is operable to be contacted by a finger of a user of the electronic device while another electrode is positioned against skin of the user. The processing unit of the electronic device is operable to determine a biological parameter, such as an electrocardiogram, of the user based on voltages at the electrodes.