SCIENCE. 



3i5 



DR. SIEMENS' ELECTRICAL FURNACE. 



At a meeting of the Society of Telegraphic Engineers, 

 Dr. Siemens gave the following description of his electri- 

 cal furnace : 



Amongst the means at our disposal for effecting the 

 fusion of highly refractory metals, and other substances, 

 none has been more fully recognized than the oxy- 

 hydrogen blast. The ingenious modification of the same 

 by M. H. Ste.-Claire Deville, known as the Deville fur- 

 nace, has been developed and applied for the fusion of 

 platinum in considerable quantities by Mr. George Mat- 

 they, F. R. S. 



The Regenerative Gas Furnace furnishes, however, 

 another means of attaining extremely high degrees of 

 heat, and this furnace is now largely used in the arts — 

 among other purposes, for the production of mild steel. 

 By the application of the open hearth process, 10 to 15 

 tons of malleable iron, containing only traces of carbon 

 or other substances alloyed with it, may be seen in a 

 perfectly fluid condition upon the open hearth of the fur- 

 nace, at a temperature probably not inferior to the melt- 

 ing-point of platinum. It may be here remarked that 

 the only building material capable of resisting such heats 

 is a brick composed of 98.5 per cent, of silica, and only 

 1.5 per cent, of alumina, iron, and lime, to bind the silica 

 together. 



In the Devdle furnace an extreme degree of heat is 

 attained by the union of pure oxygen with a rich gas- 

 eous fuel under the influence of a blast, whereas in the 

 Siemens furnace it is due to slow combustion of a poor 

 gas, potentiated, so to speak, by a process of accumu- 

 lation through heat stores or regenerators. 



The temperature attainable in both furnaces is limited 

 by the point of complete dissociation of carbonic acid and 

 aqueous vapor, which, according to Ste.-Claire Deville 

 and Bunsen, may be estimated at from 2500 to 2800° C. 

 But long 'before this extreme point has been reached, 

 combustion becomes so sluggish that the losses of heat 

 by radiation balance the production by combustion, and 

 thus prevent further increase of temperature. 



It is to the electric arc, therefore, that we must look 

 for the attainment of a temperature exceeding the point 

 of dissociation of products of combustion, and indeed 

 evidence is not wanting to prove the early application of 

 the electric arc to produce effects due to extreme elevation 

 of temperature. As early as the year 1807, Sir Humphrey 

 Davy succeeded in decomposing potash by means of an 

 electric current from a Wollaston battery of 400 elements ; 

 and in 18 to the same philosopher surprised the members 

 of the Royal Institution by the brilliancy of the electric 

 arc produced between carbon points^through the same 

 agency. 



Magneto-electric and dynamo-electric currents enable 

 us to produce the electric arc more readily and economic- 

 ally than was the case at the time of Sir Humphrey Davy, 

 and this comparatively new method has been taken ad- 

 vantage of by Messrs. Huggins, Lockyer and other physi- 

 cists, to advance astronomical and chemical research 

 with the aid of spectrum analysis. Professor Dewar, 

 quite recently, in experimenting with the dynamo-electric 

 current, has shown that in his lime tube or crucible sev- 

 eral of the metals assume the gaseous condition, as 

 demonstrated by the reversal of the lines in his spectrum, 

 thus proving that the temperature attained was not much 

 inferior to that of the sun. 



My present object is to show that the electric arc is 

 not only capable of producing a very high temperature 

 within a focus or extremely contracted space, but also 

 such large effects, with comparatively moderate expendi- 

 ture of energy, as will render it useful in the arts for fus- 

 ing platinum, iridium, steel or iron, or for affecting such 



reactions or decompositions as require for their accom- 

 plishment an intense degree of heat, coupled with freedom 

 from such disturbing influences as are inseparable from 

 a furnace worked by the combustion of carbonaceous 

 material. 



The apparatus which I employ consists of an ordinary- 

 crucible of plumbago or other highly refractory material, 

 placed in a metallic jacket or outer casing, the interven- 

 ing space being filled up with pounded charcoal or other 

 bad conductor of heat. A hole is pierced through the 

 bottom of the crucible for the admission of a rod of iron, 

 platinum or dense carbon, such as is used in electric 

 illumination. The cover of the crucible is also pierced 

 for the reception of the negative electrode, by preference 

 a cylinder of compressed carbon of comparatively large 

 dimensions. At one end of a beam supported at its cen- 

 tre is suspended the negative electrode bv means of a 

 strip of copper, or other good conductor of electricity, 

 the other end of the beam being attached to a hollow 

 cylinder of soft iron free to move vertically within a 

 solenoid coil of wire, presenting a total resistance of 

 about 50 units or ohms. By means of a sliding weight, 

 the preponderance of the beam in the direction of the 

 solenoid can be varied so as to balance the magnetic 

 force with which the hollow iron cylinderis drawn into the 

 coil. One end of the solenoid coil is connected with the 

 positive, and the other with the negative pole of the 

 electric arc, and, being a coil of high resistance, its at- 

 ractive force on the iron cylinder is proportional to the 

 electromotive force betweeen the two electrodes, or, in 

 other words, to the electrical resistance of the arc itself. 



The resistance of the arc was determined and fixed at 

 will within the limits of the source of power, by sliding 

 the weight upon the beam. If the resistance of the arc 

 should increase from any cause, the current passing 

 through the solenoid would gain in strength, and the 

 magnetic force, overcoming the counteracting weight 

 would cause the negative electrode to descend deeper 

 into the crucible; whereas, if the resistance of the arc 

 should fall below the desired limit, the weight would 

 drive back the iron cylinder within the coils, and the 

 length of the arc would increase, until the balance be- 

 tween the forces engaged had been re-established. 



The automatic adjustment of the arc is of great im- 

 portance to the attainment of advantageous results in the 

 process of electric fusion ; without it the resistance of the 

 arc would rapidly diminish with increase of temperature 

 of the heated atmosphere within the crucible, and heat 

 would be developed in the dynamo-electric machine to 

 the prejudice of the electric furnace. The sudden sink- 

 ing or change in electrical resistance of the material un- 

 dergoing fusion would, on the other hand, cause sudden 

 increase in the resistance of the arc, with a likelihood of 

 its extinction, if such self-adjusting action did not take 

 place. 



Another important element of success in electric fusion 

 consists in constituting the material to be fused the posi- 

 tive pole of the electric arc. It is well known that it is 

 at the positive pole that the heat is principally developed, 

 and fusion of the material constituting the positive pole 

 takes place even before the crucible itself is heated up to 

 the same degree. This principle of action is of course 

 applicable only to the melting of metals and other elec- 

 trical conductors, such as metallic oxides, which consti- 

 tute the materials generally operated upon in metallurgi- 

 cal processes. In operating upon non-conductive earth 

 or upon gases, it becomes necessary to provide a non- 

 destructible positive pole, such as platinum or iridium, 

 which may, however, undergo fusion, and form a little 

 pool at the bottom of the crucible. 



In this electrical furnace some time, of course, is occu- 

 pied to bring the temperature of the crucible itself up to 

 a considerable degree, but it is surprising how rapidly 

 an accumulation of heat takes place. In working wjtf) 



