310 THE ELECTEIC BDENACE. 



present moment, where other generalU^ accepted methods are out of 

 the question. The actual temperature is, of course, obtained from a 

 specially prepared table or curve, and is read from the electro-motive 

 force recorded by the thermo-couple. 



Another, somewhat crude, method of measuring furnace tempera- 

 tures, into which the personal element is liable to enter, causing an 

 error of judgment, consists in a telescope, as before, mounted on a 

 convenient stand and placed in line with a small aperture in the furnace 

 wall. Inside the tube of the telescope is located a small incandescent 

 lamp, which can be energized by one or two battery cells, and the 

 current through it, and an ammeter placed in series with it, regulated 

 by a suitable switch and rheostat. The principle upon which its action 

 depends is that which involves the apparent disappearance of the 

 tilament when raised to the same degree of incandescence as the fur- 

 nace lining and viewed against the latter as a background. If the 

 lamp be inactive, the iilament appears as a black line; at equal incan- 

 descence it becomes invisible, while if its state of incandescence be 

 above that of the furnace it assumes the appearance of a white line. 

 By regulating, therefore, the current through the lamp until the tila- 

 ment apparently disappears, its temperature is made equivalent to that 

 of the furnace, and the result is read on a specially prepared table. 

 The limit of the apparatus is 3,600° F., so that for electric-furnace 

 work its field of utility is somewhat limited. 



In the preceding paragraphs the writer has l)y no means covered the 

 entire field of development of the electric furnace, but has confined 

 himself to a brief description of those examples which serve as a 

 general type of the class to which they, respectively, belong. The 

 subject is a large one, and its comprehensive study would till a volume 

 of no mean dimensions, while its importance from a chemical and 

 metallurgical point of view must not be underrated. 



At the end of the yenv 1900 the power used in electric furnaces was 

 estimated at 225,000 horsepower, of which 185,000 horsepower were 

 employed in the manufacture of calcium carbide, 27,000 horsepower 

 in the manufacture of alumiiuim, 11,000 horsepower in that of copper, 

 while carborundum was responsible for the output of some 2,000 

 horsepower. Any gain, therefore, in the construction or working of 

 electric furnaces, however slight, or apparently worthless, provides 

 food for serious reflection, in that it ma}' he tlie means of saving large 

 sums of money annually. 



