Mercury thermometers are operated by the thermal expansion of mercury in direct proportion to the temperature. Mercury is very stable, and its coefficient of thermal expansion is constant in the thermometer.
The electronic thermometer uses the thermoelectric potential generated by the thermal sensor to determine the temperature. Due to the problem of zero drift and cold end compensation of thermal sensor, the accuracy of electronic thermometer with low level may be slightly worse.
Electronic thermometers are calibrated with mercury thermometers. So mercury thermometers are more accurate.