MAGNETISM AND ELECTRICITY 181 



The lower portion of the thermo-element, which is inserted 

 into the metal, is protected by crucible material (a clay sub- 

 stance that will resist great heat) or by a tube of pure graphite 

 with an insertion of quartz glass. In the latter case, the 

 graphite protection can only be 8 in. long, whereas in the 

 former case (for temperatures up to 2370 F.), the protection 

 tube for the thermo-element can be any desired length. The 

 latter is particularly valuable in cases where the increase 

 of temperature has to be watched while the crucible is in 

 the oven, so that it can be lifted out at the correct 

 moment. 



The thermo-element consists essentially of two wires or 

 rods of different materials, which are joined or fused together 

 at their extreme ends and exposed to the heat. These ends 

 are called the hot junction. The other extremes of the rods 

 are called the cold junction. The cold junction projects 

 into the open air and is connected to the leading wires of 

 the galvanometer by means of screws. 



The two rods of the thermo-element are of different elec- 

 trical conductivity. If, therefore, the ends of the rods at 

 the hot junction are heated, a difference of potential is pro- 

 duced, causing an electric current to flow, varying in strength 

 with the degree of the thermal difference between the cold 

 and the hot junctions, or with the intensity of the heat to 

 which the thermo-element is exposed. The relation of this 

 current to the temperature has been determined accurately 

 by experiment, and the scale of the galvanometer can there- 

 fore be divided to read directly in Fahrenheit or Centigrade 

 degrees. Thus, as soon as the thermo-element is exposed 

 to heat or cold, the electric pressure or current produced in 

 the two rods actuates the mechanism of the galvanometer, 

 and the needle of the latter indicates directly the exact 



