Page:Popular Science Monthly Volume 44.djvu/52

44 It is chiefly used at present in forming the longitudinal seams of tubes and vessels, and in filling in blow boles and other imperfections in castings. In this latter operation additional pieces of metal are fused into the openings, rendering the castings as sound and good as though they had come from the mold in perfect condition.

A method of using the electric current, substantially the same as that of Thomson, is employed by Mr. George D. Burton, and shown in operation at the exhibition by the Electrical Forging Company. Mr. Burton's object is not, however, to heat the metal simply at the line of juncture and then complete the union of the parts in one operation, but to heat a piece of metal either throughout its entire length or any particular part, and then forge it into shape by the hammer or special machines designed to produce particular forms. He uses, as in the Thomson apparatus, the alternating current transformed to one of great volume and low voltage; but instead of employing a number of converters, each adapted to the special work in hand, he makes one large one suffice, tapping this at as many points as desired. The holding device for the bars to be heated consists merely of a massive pair of copper clamps easily manipulated by the workman, and from which the work may be quickly transferred to the anvil or shaping machine.

The economy in time of electric heating is very strikingly shown where long bars and rods are heated. For instance, a round bar of tool steel, seven eighths of an inch in diameter and eleven inches long between the clamps of the machine, may be brought to a welding heat in one minute by the expenditure of thirty-two horse power. A bar of the same material, half an inch in diameter and five inches long, requires but twenty-seven and a half horse power for half a minute; while one an inch square and twelve inches long is raised to a white heat by thirty-six horse power in two minutes and a half. Generally speaking, the electric heating may be done in a tenth of the time required by the forge or furnace, and the power required is between three and four horse power per cubic inch of metal heated. The feature of electric heating already noticed of a bar becoming more highly heated at the center than at the surface when exposed freely to the air, is shown in a very convincing manner at this exhibit by fusing the core of an inch bar without it losing its shape. A consequence of this internal heating of a bar is the holding of its heat much longer than a forge-heated one, permitting of forging operations with one heat which would require two or three by the old method.

One of the most striking things in the exhibition—remarkable on account of being so entirely out of harmony with all our ideas