018 – Thermocouple

Thermoelectric effect  

Seebeck effect  

Generation of voltage by heating  

 

Introduction to:  

  • Thermometry  
  • Electric thermometer  

 

Material:  

a) 

  • Electrode for welding, made of nickel (the coating must first be removed using pliers and coarse sandpaper)  
  • Copper wire (approx. 20 cm)       

Contact voltage 20 μV/K  

or

b) 

  • Constantan wire (length approx. 20 cm; diameter approx. 0.5 mm)  
  • Iron wire (length approx. 20 cm; diameter approx. 0.5 mm)  

Contact voltage 43 μV/K  

or

c) 

  • Nickel-chromium wire (length approx. 20 cm; diameter approx. 0.5 mm)  
  • Nickel wire (length approx. 20 cm; diameter approx. 0.5 mm)  

Contact voltage 40 μV/K  

In general:Pairs of wires of two different metals A and B, e.g. also iron and zinc, are suitable  

  • Voltage meter in the millivolt range / multimeter  
  • Alligator Clips  
  • Candle / Lighter  

Setup: 

a) 

  • Remove the insulation from the copper wire and wrap it around the nickel electrode once or twice, creating a firm bond between the metals.

b) c) 

  • Twist the ends of the two wires over a length of 5 cm 
  • Connect the voltage meter to the free ends of metal A and metal B.

Execution:  

  • Heat the contact point of the metals with the candle flame. 

Observation:  

  • The voltage meter shows a contact voltage in the millivolt range.
  • As the contact point heats up, the contact voltage increases.
  • For the metal pair a) copper – nickel it is : 20 μV/K, i.e. 2 mV corresponds to a temperature increase of 100 K. 
  • When cooling, the contact voltage decreases. 

Explication:  

  • The contact voltage is due to the different exit work of electrons from different metals, the Fermi levels are of different heights.
  • The contact voltage increases in proportion to the (absolute) temperature at the contact point.

Tips:  

  • The contact between the metals can be increased by hitting the point of twisting with a hammer or soldering the wires together. 
  • With a voltage meter in the μV range, even the heat of the hand can be detected.
  • In the metal pair metal A nickel-chromium – metal B nickel, the contact voltage is about 40 μV/K (above 0 °C).