Medical thermometer and displaying temperature became available, electronic thermometers (often called "digital", because they display numeric values) have been used. Many display readings to great precision (0.1 °C or 0.2 °F, sometimes half that), but this should not be taken as a guarantee of accuracy: specified accuracy must be checked in documentation and maintained by periodical 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
Thermodynamic temperature as SPRTs, PRTs or Platinum RTDs) and monochromatic radiation thermometers. For some types of thermometer the relationship between the property observed (e.g., length of a mercury column) and temperature, is close to linear, so for most purposes a linear scale is sufficient, without point-by-point calibration. For others a calibration curve or equation is required. The mercury thermometer, invented before the thermodynamic temperature was understood, originally "defined" the temperature scale; its linearity made readings correlate well with true temperature, i.e. the "mercury" temperature scale was a close fit to the true scale. The thermodynamic temperature is a measure of the average