1. _Iron or Mild Steel._—For temperatures not exceeding 1100° C. iron or mild steel covers are cheap and efficient from the standpoint of conductivity, although liable to deteriorate owing to oxidation. The tendency to oxidise is greatly diminished by “calorising” the exterior by Ruder’s process, in which the iron is heated in a mixture of metallic aluminium and oxide of aluminium, a surface alloy being formed which resists oxidation. A result nearly as good may be obtained by smearing the surface with fine aluminium powder, and bringing to a white heat. This treatment greatly prolongs the life of an iron sheath. Some makers employ an inner steel tube round the wires, and an outer tube which comes into contact with the furnace gases, corrosion of the latter being detected before the inner tube has given way and exposed the junction. Some makers do not consider it safe to expose heated platinum to an iron surface, with only air intervening, and hence use an inner cover of silica or porcelain, which the outer iron or steel tube protects from mechanical damage. For ordinary work seamless steam or hydraulic steel tubing, with a welded end, is satisfactory; but for dipping into molten lead or other metals the tube should be bored from the solid. The great advantage of an iron or steel sheath is its mechanical strength, which protects the couple from damage in case of rough usage. 2. _Nichrom._—Certain alloys of nickel and chromium, and especially that known as Nichrom II, may be kept at 1100° C. without oxidising to any appreciable extent; and hence sheaths of this material may be used up to the temperature named. In addition to being more durable than iron, nichrom possesses the same advantages of strength and good conductivity; on the other hand, it is more costly. 3. _Molybdenum._—This metal, which possesses a melting point of about 2500° C., may be dipped in molten brass, bronze, copper, etc., without being attacked, and has been used to form the tip of a protecting-tube designed to measure the temperature of molten alloys. A junction covered only by a thin tube of molybdenum quickly attains the temperature of its surroundings. 4. _Graphite and Graphite Compositions._—Carbon has the highest melting point of all known substances, and in the form of artificial or Acheson graphite may be easily machined to any desired shape. Graphite sheaths are sometimes used for immersion in molten metals, but at 1000° C. and higher Acheson graphite oxidises easily and becomes friable. It is a good conductor of heat, but is easily broken. Compositions of natural graphite and refractory earths, such as Morgan’s “Salamander,” are inferior to pure graphite in conductivity, but are stronger and not readily oxidised, and may be used to form sheaths for temperatures up to 1400° C. or possibly higher, when penetration of furnace gases to the junction is not of moment.