measured. It is connected by wires to a resistance measuring  device inside the cockpit indicator. The instrument dial is  calibrated in degrees Fahrenheit or Celsius as desired rather  than in ohms. As the temperature to be measured changes, the  resistance of the metal changes and the resistance measuring  indicator shows to what extent.  A typical electrical resistance thermometer looks like any  other temperature gauge. Indicators are available in dual  form for use in multiengine aircraft. Most indicators are  self-compensating for changes in cockpit temperature. The  heat-sensitive resistor is manufactured so that it has a definite  resistance for each temperature value within its working  range. The temperature-sensitive resistor element is a length  or winding made of a nickel/manganese wire or other suitable  alloy in an insulating material. The resistor is protected by  a closed-end metal tube attached to a threaded plug with a  hexagonal head.  [Figure10-68]  The two ends of the winding  are brazed, or welded, to an electrical receptacle designed to  receive the prongs of the connector plug. through the galvanometer in one direction or the other. The  galvanometer pointer is actually the temperature gauge  pointer. As it moves against the dial face calibrated in degrees,  it indicates temperature. Many indicators are provided with  a zero adjustment screw on the face of the instrument. This  adjusts the zeroing spring tension of the pointer when the  bridge is at the balance point (the position at which the bridge  circuit is balanced and no current flows through the meter). The indicator contains a resistance-measuring instrument.  Sometimes it uses a modified form of the Wheatstone- bridge circuit. The Wheatstone-bridge meter operates on the  principle of balancing one unknown resistor against other  known resistances. A simplified form of a Wheatstone- bridge circuit is shown in  Figure 10-69 . Three equal values  of resistance  [Figure 10-69A, B,  and  C]  are connected  into a diamond shaped bridge circuit. A resistor with an  unknown value  [Figure 10-69D]  is also part of the circuit.  The unknown resistance represents the resistance of the  temperature bulb of the electrical resistance thermometer  system. A galvanometer is attached across the circuit at  points X and Y. When the temperature causes the resistance of the bulb to  equal that of the other resistances, no potential difference  exists between points X and Y in the circuit. Therefore, no  current flows in the galvanometer leg of the circuit. If the  temperature of the bulb changes, its resistance also changes,  and the bridge becomes unbalanced, causing current to flow  The resistance bulb sensing portion of the ratiometer electric  resistance thermometer is essentially the same as described  above. The circuit contains a variable resistance and a fixed  resistance to provide the indication. It contains two branches  for current flow. Each has a coil mounted on either side of  the pointer assembly that is mounted within the magnetic  field of a large permanent magnet. Varying current flow  through the coils causes different magnetic fields to form,  which react with the larger magnetic field of the permanent  magnet. This interaction rotates the pointer against the dial  face that is calibrated in degrees Fahrenheit or Celsius, giving  a temperature indication.  [Figure 10-70] Another way of indicating temperature when employing  an electric resistance thermometer is by using a ratiometer.  The Wheatstone-bridge indicator is subject to errors from  line voltage fluctuation. The ratiometer is more stable and  can deliver higher accuracy. As its name suggests, the  ratiometer electrical resistance thermometer measures a  ratio of current flows.  Figure 10-69.  The internal structure of an electric resistance  thermometer indicator features a bridge circuit, galvanometer,  and variable resistor, which is outside the indicator in the form of  the temperature sensor.  Figure 10-68.  An electric resistance thermometer sensing bulb.  10-38 Ratiometer Electrical Resistance Thermometers + − Heat-sensitive element or bulb 14Volts Indicator A B M L X C Cal. res. Y D