The details of construction of the furnaces which are used for metal for castings are more or less different, but they are not of particular interest to us. The working of all is similar and a general description should suffice. [Illustration: DRAWING OF THE SNYDER ELECTRIC FURNACE ] [Illustration: SMALL SNYDER ELECTRIC FURNACE TAPPING ] Whether starting with furnace cold or hot, materials in molten or in the more usual “cold” form are charged on the shallow hearth of the furnace. The charging doors are closed, the current is turned on and the carbon electrodes are lowered until an arc is struck between the upper electrodes and the metal on the hearth, which in some way is made to connect with the negative electrodes. In one or two of the types mentioned the arc plays between the carbons, all of which are above the bath. At first there are great fluctuations in the current intensity because of the uneven surface presented by scrap steel on the hearth. In a short time, however, the current steadies. The intense heat of the arcs soon brings cold steel to a molten condition. Occasional attention from the attendant is necessary to see that the melting is even and that any outlying pieces of steel are pushed to the center where they must melt. [Illustration: SKETCH OF THE HEROULT THREE-PHASE ELECTRIC FURNACE There are three electrodes, all of them above the bath. Only two show here. ] In the basic-lined furnaces lime is usually charged with the cold steel. With the iron oxide which is added from time to time this forms a highly oxidizing slag, which, after it takes the phosphorus from the metal, is skimmed off. As you will remember, the other processes stop at this point, little further refining being possible. In the electric furnace, however, the sulphur, also, can be reduced to almost any desired amount by use of a further addition of lime, and greater heat. Not only can the sulphur be reduced to very small percentages but the over-oxidized bath can be brought to neutral condition and the green or black slag made white with return of its manganese and iron to the bath. This is accomplished by addition of small amounts of powdered coke or coal. The whole process is under very accurate control. With a practically white slag, which is the signal that the deoxidation of the bath is complete, and the sulphur reduced, the steel is ready to pour provided it is hot enough. Tests of this are usually made either by pouring a little of the steel from a small ladle and observing its fluidity or by observing the quickness with which the end of an iron bar is melted off when plunged into the metal in the furnace. [Illustration: THE HEROULT ELECTRIC FURNACE ]