TAILIEUCHUNG - THE ELECTRIC FURNACEMakingthe197O '. the equation JKTP o

THE ELECTRIC FURNACE Making the 197 O '. the equation JKT P o"Vy temperature distribution for minimum gives ' T=Q t = ^e ara bola p. To find loss, solve for T in -TTT = , and substitute in (12), obtaining the parabola P. When - is greater or less than H, the tem- or P" respectively. perature distribution is given by In any problem involving the design of electrodes, the temperature difference between the hot and cold ends of the elecf P trode and the kilowatts to be absorbed in the furnace will be given. From be made as high the value of the power the voltage would then and the current as low as practicable. From formula (9) compute the proportion of the section to the The length, which should be as short as. | THE ELECTRIC FURNACE 197 13 Making T 0 gives t the parabola p. To find the temperature distribution for minimum loss solve for T in the equation jKT and substitute in 12 obtaining 1J 2 JU the parabola p. When -T- is greater or less than H the ternA perature distribution is given by pl or P 1 respectively. In any problem involving the design of electrodes the temperature difference between the hot and cold ends of the electrode and the kilowatts to be absorbed in the furnace will be given. From the value of the power the voltage would then be made as high and the current as low as practicable. From formula 9 compute the proportion of the section to the length. The length which should be as short as possible will be determined by the thickness of the walls of the furnace. Having fixed the length the section is then obtained from the ratio of the section to the length. The two remaining factors which must be known are the values of the heat and electrical conductivities of carbon and graphite the only two substances used for electrodes in resistance furnaces. These values have not yet been determined accurately for high temperatures but the mean values have been determined by Hering between 100 c. and 900 The method of determining heat conductivity depends on the demonstration above. Tur TUT If in equation 13 z o then and lĩ In order to measure a conducting rod of length L and section embedded in a nonconducting material is heated by a measured amount of electrical energy and the temperature T measured at the center. In order to have no heat pass out the sides of the rod it is surrounded by a number of similar rods at the same temperature as the one measured. The electrical conductivity is obtained from the ammeter and voltmeter read-9 Trans. Am. Electrochem. Soc. 16 317 and 315 1909 . 198 APPLIED ELECTROCHEMISTRY ings and the dimensions. The values in Table 22 have been obtained by this The units are centimeters gram calories and ohms and centigrade .

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