(A resistance thermometer is made of metal, usually platinum, while a thermistor is made of ceramic or polymer.) Resistance thermometers and thermistors are generally used in two ways. First, they can be used as thermometers: By measuring the resistance, the temperature of the environment can be inferred. Second, they can be used in conjunction with Joule heating (also called self-heating): If a large current is running through the resistor, the resistor's temperature rises and therefore its resistance changes. Therefore, these components can be used in a circuit-protection role similar to fuses, or for feedback in circuits, or for many other purposes. In general, self-heating can turn a resistor into a nonlinear and hysteretic circuit element. For more details see Thermistor#Self-heating effects. If the temperature "T" does not vary too much, a linear approximation is typically used: where formula_16 is called the "temperature coefficient of resistance", formula_17 is a fixed reference temperature (usually room temperature), and formula_18 is the resistance at temperature formula_17. The parameter formula_16 is an empirical parameter fitted from measurement data. Because the linear approximation is only an approximation, formula_16 is different for different reference temperatures. For this reason it is usual to specify the temperature that formula_16 was measured at with a suffix, such as formula_23, and the relationship only holds in a range of temperatures around the reference. The temperature coefficient formula_16 is typically +3×10−3 K−1 to +6×10−3 K−1 for metals near room temperature. It is usually negative for semiconductors and insulators, with highly variable magnitude. Just as the resistance of a conductor depends upon temperature, the resistance of a conductor depends upon strain. By placing a conductor under tension (a form of stress that leads to strain in the form of stretching of the conductor), the length of the section of conductor under tension increases and its cross-sectional area decreases. Both these effects contribute to increasing the resistance of the strained section of conductor. Under compression (strain in the opposite direction), the resistance of the strained section of conductor decreases. See the discussion on strain gauges for details about devices constructed to take advantage of this effect. Some resistors, particularly those made from semiconductors, exhibit "photoconductivity", meaning that their resistance changes when light is shining on them. Therefore, they are called "photoresistors" (or "light dependent resistors").