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