20 0 -20 -40 -60 40 60 100 140 120 80 F 20 0 -20 -40 40 60 C −40 −30 −20 −10 0 10 20 30 40 50 60 70 80 90 100 110 120 Bimetallic temperature gauge  Bimetallic coil of bonded metals with  dissimilar coefficients of expansion Figure 10-67.  A bimetallic outside air temperature gauge and its  installation on a light aircraft.  Figure 10-66.  A bimetallic temperature gauge works because of the  dissimilar coefficients of expansion of two metals bonded together.  When bent into a coil  cooling or heating causes the dissimilar metal  coil to tighten  or unwind  moving the pointer across the temperature  scale on the instrument dial face.  10-37 Electrical Resistance Thermometer Electrical Temperature Measuring Indication directly with temperature. By filling a sensing bulb with such  a volatile liquid and connecting it to a bourdon tube  the tube  causes an indication of the rising and falling vapor pressure  due to temperature change. Calibration of the dial face in  degrees Fahrenheit or Celsius  rather than psi  provides a  temperature reading. In this type of gauge  the sensing bulb  is placed in the area needing to have temperature measured.  A long capillary tube connects the bulb to the bourdon tube in  the instrument housing. The narrow diameter of the capillary  tube ensures that the volatile liquid is lightweight and stays  primarily in the sensor bulb. Oil temperature is sometimes  measured this way. The use of electricity in measuring temperature is very  common in aviation. The following measuring and indication  systems can be found on many types of aircraft. Certain  For most metals  electrical resistance changes as the  temperature of the metal changes. This is the principle upon  which a resistance thermometer operates. Typically  the  electrical resistance of a metal increases as the temperature  rises. Various alloys have a high temperature-resistance  coefficient  meaning their resistance varies significantly  with temperature. This can make them suitable for use in  temperature sensing devices. The metal resistor is subjected  to the fluid or area in which temperature needs to be  The principle parts of the electrical resistance thermometer  are the indicating instrument  the temperature-sensitive  element (or bulb)  and the connecting wires and plug  connectors. Electrical resistance thermometers are used  widely in many types of aircraft to measure carburetor air   oil  free air temperatures  and more. They are used to measure  low and medium temperatures in the –70 °C to 150 °C range. temperature ranges are more suitably measured by one or  another type of system. 