A wearable device, comprising: a housing; first (840, 910) and second (845, 915)_electrical contacts protruding from the housing, wherein the first (840, 910) and second (845, 915) electrical contacts are configured to contact skin at an external body surface when the housing is mounted on the external body surface; and electronics (830, 900) disposed in the wearable device, wherein the electronics comprises: a capacitor, wherein the capacitor has a specified capacitance; an electronic switch in series with the capacitor, wherein the series combination of the capacitor and the electronic switch is electrically connected between the first (840, 910) and second (845, 915) electrical contacts; a resistance sensor electrically coupled to the second electrical contact (845, 915) via the series combination of the capacitor and the electronic switch, wherein the resistance sensor is configured to obtain a measurement relating to a resistance of the skin between the first (840, 910) and second (845, 915) electrical contacts when the electronic switch is closed and the wearable device is mounted to the external body surface; and a capacitance sensor electrically coupled to the second electrical contact (845, 915) via the series combination of the capacitor and the electronic switch and in parallel with the resistance sensor, wherein the capacitance sensor is configured to obtain a measurement relating to a capacitance of the skin between the first (840, 910) and second (845, 915) electrical contacts when the electronic switch is open and the wearable device is mounted to the external body surface. a housing; first (840, 910) and second (845, 915)_electrical contacts protruding from the housing, wherein the first (840, 910) and second (845, 915) electrical contacts are configured to contact skin at an external body surface when the housing is mounted on the external body surface; and electronics (830, 900) disposed in the wearable device, wherein the electronics comprises: a capacitor, wherein the capacitor has a specified capacitance; an electronic switch in series with the capacitor, wherein the series combination of the capacitor and the electronic switch is electrically connected between the first (840, 910) and second (845, 915) electrical contacts; a resistance sensor electrically coupled to the second electrical contact (845, 915) via the series combination of the capacitor and the electronic switch, wherein the resistance sensor is configured to obtain a measurement relating to a resistance of the skin between the first (840, 910) and second (845, 915) electrical contacts when the electronic switch is closed and the wearable device is mounted to the external body surface; and a capacitance sensor electrically coupled to the second electrical contact (845, 915) via the series combination of the capacitor and the electronic switch and in parallel with the resistance sensor, wherein the capacitance sensor is configured to obtain a measurement relating to a capacitance of the skin between the first (840, 910) and second (845, 915) electrical contacts when the electronic switch is open and the wearable device is mounted to the external body surface. a capacitor, wherein the capacitor has a specified capacitance; an electronic switch in series with the capacitor, wherein the series combination of the capacitor and the electronic switch is electrically connected between the first (840, 910) and second (845, 915) electrical contacts; a resistance sensor electrically coupled to the second electrical contact (845, 915) via the series combination of the capacitor and the electronic switch, wherein the resistance sensor is configured to obtain a measurement relating to a resistance of the skin between the first (840, 910) and second (845, 915) electrical contacts when the electronic switch is closed and the wearable device is mounted to the external body surface; and a capacitance sensor electrically coupled to the second electrical contact (845, 915) via the series combination of the capacitor and the electronic switch and in parallel with the resistance sensor, wherein the capacitance sensor is configured to obtain a measurement relating to a capacitance of the skin between the first (840, 910) and second (845, 915) electrical contacts when the electronic switch is open and the wearable device is mounted to the external body surface.