One problem with mercury and dial thermometers is that they take a while to react to temperature changes. Electronic thermometers don't have that problem: you simply touch the thermometer probe onto the object whose temperature you want to measure and the digital display gives you an instant temperature reading.
Electronic thermometers work in an entirely different way to mechanical ones that use lines of mercury or spinning pointers. They're based on the idea that the resistance of a piece of metal (the ease with which electricity flows through it) changes as the temperature changes. As metals get hotter, atoms vibrate more inside them, it's harder for electricity to flow, and the resistance increases. Similarly, as metals cool down, the electrons move more freely and the resistance goes down. (At temperatures close to absolute zero, the lowest theoretically possible temperature of −273.15°C or −459.67°F, resistance disappears entirely in a phenomenon called superconductivity.)
Photo: A compact electronic medical thermometer. You put the metal probe (left) in your mouth, or somewhere else on your body, and read the temperature off the LCD display.
An electronic thermometer works by putting a voltage across its metal probe and measuring how much current flows through it. If you put the probe in boiling water, the water's heat makes electricity flow through the probe less easily so the resistance goes up by a precisely measurable amount. A microchip inside the thermometer measures the resistance and converts it into a measurement of temperature.
The main advantage of thermometers like this is that they can give an instant reading in any temperature scale you like—Celsius, Fahrenheit, or whatever it happens to be. But one of their disadvantages is that they measure the temperature from moment to moment, so the numbers they show can fluctuate quite dramatically, sometimes making it difficult to take an accurate reading.