Two wire: in the thermocouple at both ends of the connection to elicit resistance signal way is called a wire two wire: this kind of wire method is very simple, there must be lead resistance r, but because of the connection wire r size and material of the wire and the length of the factors, so this method is only applicable to lead at a lower accuracy of measurement. Three-wire system: at one end of the thermocouple roots connect a wire, the other end connected to the two lead way called three wire system, the way which is equipped with a bridge, can be better to eliminate the influence of the lead resistance, the lead resistance is very commonly used in industrial process control. Four wire: on both ends of the thermocouple root way each connecting two wire called a four wire system, including two thermocouples provide constant current I, lead to voltage signal into R U, again through the other two root lead to guide U to secondary instrument. Is this way of lead can completely eliminate the resistance of the lead, it is mainly used for high precision temperature detection. Thermocouple used three wire connection. Using three wire system is to eliminate the measuring error caused by the connection wire resistance. This is because the measuring circuit of thermocouple is usually unbalanced bridge. Thermocouple as a bridge bridge arm resistance, its connection wire ( From the thermocouple is controlled by the) This part also become a part of the bridge arm resistance, resistance is unknown and change with environment temperature, cause measurement error. Using three wire system, will lead a received the power supply end of the bridge, the remaining two received thermocouple respectively the bridge arm and the adjacent bridge arm, this eliminates the wire line resistance measurement error. Industry generally adopted three wire connection. Thermocouple is millivolt signal, there is no this problem.