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These two pyrometers have the disadvantage of needing to be con-
stantly watched, and many inlays have been spoiled with this style of
pyrometer furnace because the heat has been carried beyond the point
required to fuse the jjorcclain, or the inlay is left at the maximum heat
required to fuse it for too great a length of time. If this style of pyro-

FiG. 128.

meter is used, the operator should pay strict attention to the instrument
when the heat becomes sufficient to hard-biscuit the porcelain.
The Roach automatic furnace (Fig. 128) is constructed on a dififer-
ent principle. A small lead plug makes the connection just below the
muffle and conijiletes the circuit. When sufficient heat is obtained the
plug melts and breaks the circuit. The degree of heat is regulated by
the distance the plug is placed from the muffle. A series of figures are

114