gyroscopes and traveling-wave tubes (TWTs), need to have constant field over a wide temperature range. The reversible temperature coefficient (RTC) of B is defined as: To address these requirements, temperature compensated magnets were developed in the late 1970s. For conventional SmCo magnets, B decreases as temperature increases. Conversely, for GdCo magnets, B increases as temperature increases within certain temperature ranges. By combining samarium and gadolinium in the alloy, the temperature coefficient can be reduced to nearly zero. Electrical resistance The temperature dependence of electrical resistance and thus of electronic devices (wires, resistors) has to be taken into account when constructing devices and circuits. The temperature dependence of conductors is to a great degree linear and can be described by the approximation below. where just corresponds to the specific resistance temperature coefficient at a specified reference value (normally T = 0 °C) That of a semiconductor is however exponential: where is defined as the cross sectional area and and are coefficients determining the shape of the function and the value of resistivity at a given temperature. For both, is referred to as the temperature coefficient of resistance (TCR). This property is used in devices such as thermistors. Positive temperature coefficient of resistance A positive temperature coefficient (PTC) refers to materials that experience an increase in electrical resistance when their temperature is raised. Materials which have useful engineering applications usually show a relatively rapid increase with temperature, i.e. a higher coefficient. The higher the coefficient, the greater an increase in electrical resistance for a given temperature increase. A PTC material can be designed to reach a maximum temperature for a given input voltage, since at some point any further increase in temperature would be met with greater electrical resistance. Unlike linear resistance heating or NTC materials, PTC materials are inherently self-limiting. On the other hand, NTC material may also be inherently self-limiting if constant current power source is used. Some materials even have exponentially increasing temperature coefficient. Example of such a material is PTC rubber. Negative temperature coefficient of resistance A negative temperature coefficient (NTC) refers to materials that experience a decrease in electrical resistance when their temperature is raised. Materials which have useful engineering applications usually show a relatively rapid decrease with temperature, i.e. a lower coefficient. The lower the coefficient, the greater a decrease in electrical resistance for a given temperature increase. NTC materials are used to create inrush current