Back of indicating instrument Connectors Thermocouple leads Black Voltmeter inside forms cold junction Hot junction Constantan (chrome on turbine engine) Copper or iron (alumel on turbine  engines) Typical Thermocouple Gasket type thermocouple Engine cylinder spark plug bore A B Figure 10-72.  A cylinder head temperature thermocouple with a  gasket type hot junction is made to be installed under the spark plug  or a cylinder hold down nut of the hottest cylinder (A). A bayonet  type thermocouple is installed in a bore in the cylinder wall (B).  Figure 10-71.  Thermocouples combine two unlike metals that cause current flow when heated.  chosen for the thermocouple installation is the one that runs  the hottest under most operating conditions. The location of  this cylinder varies with different engines.  [Figure 10-72] The cold junction of the thermocouple circuit is inside the  instrument case. Since the electromotive force set up in the  circuit varies with the difference in temperature between  the hot and cold junctions, it is necessary to compensate the  indicator mechanism for changes in cockpit temperature  which affect the cold junction. This is accomplished by using  a bimetallic spring connected to the indicator mechanism.  This actually works the same as the bimetallic thermometer  described previously. When the leads are disconnected from  the indicator, the temperature of the cockpit area around  the instrument panel can be read on the indicator dial.  [Figure 10-73]  Numeric LED indictors for CHT are also  common in modern aircraft. EGT is a critical variable of turbine engine operation.  The EGT indicating system provides a visual temperature  indication in the cockpit of the turbine exhaust gases as  they leave the turbine unit. In certain turbine engines, the  temperature of the exhaust gases is measured at the entrance  to the turbine unit. This is referred to as a turbine inlet  temperature (TIT) indicating system. 10-40 Turbine Gas Temperature Indicating Systems