mounted as small dots in a matrix on a thin plastic spatula with a protective transparent cover. This is placed under the patient's tongue. After a short time the spatula is removed and it can be seen which dots have melted and which have not: the temperature is taken as the melting temperature of the last dot to melt. These are cheap disposable devices and avoid the need for sterilizing for re-use. Liquid crystal A liquid crystal thermometer contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change color to indicate different temperatures. Electronic Since compact and inexpensive methods of measuring and displaying temperature became available, electronic thermometers (often called digital, because they display numeric values) have been used. Many display readings with a resolution of only 0.1 °C (.2 °F), but this should not be taken as a guarantee of accuracy: the specified accuracy must be checked in the documentation and maintained by periodic recalibration. A typical inexpensive electronic ear thermometer for home use has a displayed resolution of 0.1 °C, but a stated accuracy within ±0.2 °C (±0.35 °F) when new. The first electronic clinical thermometer, invented in 1954, used a flexible probe that contained a Carboloy thermistor. Types of Digital Thermometer Resistance temperature detectors (RTDs) RTDs are wire windings or other thin film serpentines that exhibit changes in resistance with changes in temperature. They measure temperature using the positive temperature coefficient of electrical resistance of metals. The hotter they become, the higher the value of their electrical resistance. Platinum is the most commonly used material because it is nearly linear over a wide range of temperatures, is very accurate, and has a fast response time. RTDs can also be made of copper or nickel. Advantages of RTDs include their stable output for long periods of time. They are also easy to calibrate and provide very accurate readings. Disadvantages include a smaller overall temperature range, higher initial cost, and a less rugged design Thermocouples Thermocouples are accurate, highly sensitive to small temperature changes, and quickly respond to changes to the environment. They consist of a pair of dissimilar metal wires joined at one end. The metal pair generates a net thermoelectric voltage between their opening and according to the size of the temperature difference between the ends. •Advantages of thermocouples include their high accuracy and reliable operation over an extremely wide range of temperatures. They are also