THE ELECTRIC FURNACE. 309 



which Hows down ])etween the electrodes and is maintained at constant 

 leA'el l)y the overflow ontlets o o. 



As pointed out 1)y Mr. Carl Hering-, the lire brick or other refrac- 

 tory lining- of all furnaces when heated becomes a conductor after the 

 manner of a Nernst lamp glower. Though unavoidable, this is a con- 

 tingency which, with furnaces of the resistance type at least, must be 

 taken into account, in that it increases the total conductivity of the 

 device and necessitates a corresponding increase in the working cur- 

 rent. He further points out that the heat thus conmuuiicated is not 

 lost, except in a small degree, consequent upon the decrease in the 

 thickness of the nonconducting walls and the diminution of their heat- 

 conserving qualities. 



Equally important with the selection of an easily regulated and com- 

 paratively invariable electric furnace, ranks the question of tempera- 

 ture determination. At the enormous temperatures developed in the 

 electric furnace all previously known methods of temperature measure- 

 ment, whether by thermometer or pyrometer, desert us, in that the 

 constituent parts of these various apparatuses will not stand the direct 

 application of such terrific heat. Here again stern necessity has been 

 the means of inspiring investigators to action, with a view to discov- 

 ering some efficient method for the measurement or comparison of 

 these high temperatures. 



Fery's suggested method for determining the high temperatures 

 usually encountered in electric furnaces consists in a practical applica- 

 tion of Stefan's law, which is to the effect that the radiation of an 

 absolutely black body is proportional to the fourth power of its abso- 

 lute temperature. Kirchoff' has proved that the interior of an inclosure 

 of which the walls are at a uniform temperature is equivalent to an 

 "absolutely Idack l)()dy," i. e., a l)ody which absorbs all the heat 

 imparted to it, giving it out again bj' radiation, and not l)y reflection. 

 In this connection, therefore, the interior wall of an electric or other 

 inclosed furnace may be regarded as an absolutely black body, a small 

 aperture in wdiich docs not materially aft'ect the conditions governing 

 this definition. 



Fery\s practical application of this law to the measurement of fur- 

 nace temperatures consists in a species of telescope with a fluorspar 

 objective. This telescope is placed in line with the aperture in the fur- 

 nace wall, and, receiving the heat radiated therefrom, concentrates it 

 upon a small thermo-couple. By an inner diaphragm, regulating the 

 number of rays which reach the thermo-couple, the device is rendered 

 independent of its distance from tlie furnace wall. The fluorspar 

 objective, by its absorption of radiant heat, reduces the sensitiveness 

 of the arrangement by about 10 per cent; l)ut, notwithstanding this, it 

 has proved of extreme utility, owing to its enormous range and its 

 applical)ility in such cases as those with which we are dealing at the 



