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 quick test based on thermo chromic colours a liquid crystal thermometer contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change color to indicate different temperatures. electronic electronic clinical thermometer 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 well-suited for making automated measurements both inexpensive and durable. •disadvantages include errors caused by their use over an extended period of time, and that two temperatures are required to make measurements. thermocouple materials are subject to corrosion, which can affect the thermoelectric voltage thermistor thermistor elements are the most sensitive temperature sensors available. a thermistor is a semiconductor device with an electrical resistance that is proportional to temperature. there are two types of products. •negative temperature coefficient (ntc) devices are used in temperature sensing and are the most common type of thermistor. ntcs have temperatures that vary inversely with their resistance, so that when the temperature increases, the resistance decreases, and vice versa. ntcs are constructed from oxides of materials such as nickel, copper, and iron. • positive temperature coefficient (ptc) devices are used in electric current control. they function in an opposite manner than ntc in that the resistance increases as temperature increases. ptcs are constructed from thermally sensitive silicons or polycrystalline ceramic materials. • there are several advantage and disadvantages to using an ntc thermistor thermometer. • advantages include their small size and high degree of stability. ntcs are also long lasting and very accurate. • disadvantages include their non-linearity, and unsuitability for use in extreme temperatures contact some electronic thermometers may work by contact (the electronic sensor is placed in the location where temperature is to be measured, and left long enough to reach equilibrium). these typically reach equilibrium faster than mercury thermometers; the thermometer may beep when equilibrium has been reached, or the time may be specified in the manufacturer's documentation. remote other electronic thermometers work by remote sensing: an infrared sensor responds to the radiation spectrum emitted from the location. although these are not in direct contact with the area being measured, they may still contact part of the body (a thermometer which senses the temperature of the eardrum without touching it is inserted into the ear canal). to eliminate the risk of patient cross-infection, disposable probe covers and single-use clinical thermometers of all types are used in clinics and hospitals. accuracy according to a 2001 research, electronic thermometers on the market significantly underestimate higher temperatures and overestimate lower temperatures. the researchers conclude that "the current generation of electronic, digital clinical thermometers, in general, may not be sufficiently accurate or reliable to replace the traditional glass/mercury thermometers" basal thermometer a basal thermometer is a thermometer used to take the basal (base) body temperature, the temperature upon waking. basal body temperature is much less affected than daytime temperature by environmental factors such as exercise and food intake. this allows small changes in body temperature to be detected glass oral thermometers typically have markings every 0.1 °c or 0.2 °f. basal temperature is stable enough to require accuracy of at least 0.05 °c or 0.1 °f, so special glass basal thermometers are distinct from glass oral thermometers. digital thermometers which have sufficient resolution (0.05 °c or 0.1 °f is sufficient) may be suitable for monitoring basal body temperatures; the specification should be checked to ensure absolute accuracy, and thermometers (like most digital instruments) should be calibrated at specified intervals. if only the variation of basal temperature is required, absolute accuracy is not so important so long as the readings do not have large variability (e.g., if real temperature varies from 37.00 °c to 37.28 °c, a thermometer which inaccurately but consistently reads a change from 37.17 °c to 37.45 °c will indicate the magnitude of the change). some digital thermometers are marketed as "basal thermometers" and have extra features such as a larger display, expanded memory functions, or beeping to confirm the thermometer is placed properly. smart and wearable thermometers a smart thermometer is able to transmit its readings so that they can be collected, stored and analysed. wearable thermometers can provide continuous measurement but it is difficult to measure core body temperature in this way.