An electronic watch, comprising: a housing; a crown comprising: a crown body; and a shaft connected to the crown body and extending through a wall of the housing; a carrier coupled to the housing and comprising: a light-transmissive member; and an opaque mask positioned on the light-transmissive member and defining at least one light-transmissive window in a central portion of the carrier; a transparent cover connected to the housing; a touch-sensitive display at least partially within the housing and viewable through the transparent cover; a first arc-shaped electrode positioned along a first peripheral portion of the carrier and configured to measure a first voltage, the first arc-shaped electrode extending at least partially around a central portion of the carrier; a second arc-shaped electrode positioned along a second peripheral portion of the carrier and configured to measure a second voltage, the second arc-shaped electrode extending at least partially around the central portion of the carrier; a third electrode on the crown body and configured to measure a third voltage; an optical sensor subsystem positioned below the central portion of the carrier and configured to detect a biological parameter through the at least one light transmissive window in the central portion of the carrier; and a processor within the housing and operationally connected to the first arc-shaped electrode, the second arc-shaped electrode, and the third electrode; wherein: the processor is configured to determine an electrocardiogram using the first voltage and second voltage, and the third voltage; and the touch-sensitive display is configured to display the electrocardiogram. a housing; a crown comprising: a crown body; and a shaft connected to the crown body and extending through a wall of the housing; a crown body; and a shaft connected to the crown body and extending through a wall of the housing; a carrier coupled to the housing and comprising: a light-transmissive member; and an opaque mask positioned on the light-transmissive member and defining at least one light-transmissive window in a central portion of the carrier; a light-transmissive member; and an opaque mask positioned on the light-transmissive member and defining at least one light-transmissive window in a central portion of the carrier; a transparent cover connected to the housing; a touch-sensitive display at least partially within the housing and viewable through the transparent cover; a first arc-shaped electrode positioned along a first peripheral portion of the carrier and configured to measure a first voltage, the first arc-shaped electrode extending at least partially around a central portion of the carrier; a second arc-shaped electrode positioned along a second peripheral portion of the carrier and configured to measure a second voltage, the second arc-shaped electrode extending at least partially around the central portion of the carrier; a third electrode on the crown body and configured to measure a third voltage; an optical sensor subsystem positioned below the central portion of the carrier and configured to detect a biological parameter through the at least one light transmissive window in the central portion of the carrier; and a processor within the housing and operationally connected to the first arc-shaped electrode, the second arc-shaped electrode, and the third electrode; wherein: the processor is configured to determine an electrocardiogram using the first voltage and second voltage, and the third voltage; and the touch-sensitive display is configured to display the electrocardiogram.