In all cases it must be borne in mind that the indications only apply to black-body conditions. If a block of steel be sighted inside a furnace, and then be removed to the exterior and again sighted, the external reading will be much less than the internal, owing to the inferior radiating power of the surface, which now derives no assistance from the furnace. All readings should therefore be taken whilst the object is still in the furnace, or (as in taking the temperature of molten metal in a ladle) a fireclay tube with a closed end inserted in the mass may be used, and readings taken through the open end. Statements are sometimes made that the difference between external readings and black-body readings is constant for a given surface, and that the one may be translated into the other; but this is true only for unchanging surfaces, such as platinum, and seldom applies to ordinary working surfaces. As black-body conditions are so easy to ensure, it is simpler and safer always to arrange to take observations under such conditions, rather than to trust a relation seldom constant in practice. When using a radiation pyrometer for a number of furnaces, fireclay tubes, closed at one end, may be inserted in each, so that the closed end terminates at the working spot, the open end being left flush with the exterior of the furnace. The diameter of such tubes will depend upon the length and also upon the make of the pyrometer; in all cases the image of the closed end must be large enough to overlap the receiving junction or spiral. Information on this point can always be obtained from the makers, or can be discovered by trial with openings of known diameter. When using the pyrometer to obtain temperatures in the interior of the tube of an electric furnace, such as that illustrated in fig. 29, a solid object, such as a short fireclay cylinder, or a piece of graphite, should be placed in the middle of the tube, and focused on the junction.