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the cover is a hole through which the gases issue as they are driven off by the heat in the furnace. The electrodes consist of carbon rods 24 inches in diameter, and nine of them are

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attached to each electrode rod. The attachment, as shown in the figure, consists of metal, which is cast around the ends of the carbons and of the electrode rod, so as to form a conductor, which is in perfect contact with the whole of the parts connected. The metal from which this attachment is cast is selected so as to be suitable for the particular alloy to be treated in the furnace, as, although in comparison with the actual charge, it is not subjected to a very intense heat, the metal of which it is formed is found to exercise a slight influence upon the furnace product.

"Here, then, we have a covered furnace provided at its ends with two electrodes, which are capable of being easily moved apart. The application of the furnace is as follows: The two electrode rods are connected to the two leads of the dynamo, the cable joined to the positive pole sending the current into the left-hand electrode, whence it travels through the charge, where it meets with the resistance which causes the heat, and passes by way of the right-hand electrode into the cable connected to the negative pole of the dynamo. Resistance is caused by the particles of the carbon which are mixed in the charge. Thus, the current acts independently of the structure of the furnace itself, acting simply on the charge which is in immediate contact with the carbon, by means of which the heat is produced. The heat

developed is enormous, and charcoal is found the most convenient form of non-conductor for a protective lining, but the intensity of the heat converts it in a short time into graphite, so that it loses its non-conducting properties. To renew the charcoal frequently is expensive, and the difficulty is overcome by dipping the partially graphitised charcoal into limed-water, the lime being found to render the charcoal again sufficiently non-conducting.

"When charging a cold furnace, some charcoal is put into the bottom, and lightly beaten down till it forms a firm bottom of sufficient thickness. The electrodes are run inwards till their ends nearly meet in the furnace, and the charge is then put into the centre of the furnace, and surrounding the electrode ends. The charge for copper alloy consists of 300 lbs. of materials, about one-fourth being aluminium ore broken up small, a good deal of which is supplied by broken-up slag taken from a former heat, about 4 per cent of carbon, and the balance is granulated copper.

"The ore employed is sometimes corundum, also called sapphire and ruby. The dull crystals are termed corundum, and the gray and black varieties emery. It is essentially alumina Al,0. At the present time the ore employed in England is bauxite, the composition of a sample of which is as follows:

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"This was found to answer equally well, at a fraction of the cost of corundum. The charge while being inserted is prevented from spreading out laterally (in which case some portions of it might be too distant from the electrodes) by two pieces of sheet-iron connected by a distance piece, which

are placed one on either side of the electrodes; the charge is put in between them, and the charcoal is then filled in between the sheet-iron and the sides of the furnace, after which the sheet-iron is withdrawn, the top of the charge covered, and the ends of the furnace filled with charcoal to the depth of a few inches above the charge and the electrodes. The iron cover is then luted on with fire-clay. A current of 2000 to 3000 amperes is applied at first, and, as the charge becomes fused, the resistance diminishes. The electrodes are then drawn farther apart, the speed of the engine increased, and a current of 5000 amperes flows through the charge, until the whole of the ore and metal are fused. tion requires about one hour and three-quarters. It is assumed that the aluminium and copper unite when both are in the gaseous state, and the Company claim that by this means a completeness of union between the constituents of the alloy, and a homogeneity and strength are obtained superior to alloys formed in any other way."

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The rich alloys prepared in the electrical furnace just described are used for making aluminium-bronze, brass, and iron, by melting them with copper, tin, zinc, etc., in a crucible in an ordinary furnace, instead of using pure metallic aluminium, and it is found by this means that more perfect and uniform alloys are obtained.

ALUMINIUM-BRASS

§ 80. This alloy is prepared by adding spelter to aluminiumbronze, or by melting together various proportions of ordinary brass and aluminium-bronze. The greater the proportion of the bronze present in admixture, the harder and stronger the brass becomes. Alloys containing copper, zinc, and aluminium between the following limits—

Copper
Zinc
Aluminium.

67 to 71

31 to 25

1 to 3

and combined in different proportions, have a tensile

strength varying from 13 to 30 tons per square inch. Alloys with 56 to 57 per cent copper and 40 to 42 per cent zinc approach nearer 31 tons tensile strength. Mr. Robey states that aluminium-brass is composed of of 10 per cent aluminium-bronze, copper, and spelter.

Aluminium-brass is close-grained, homogeneous, and tough, with a useful tensile strength, and great elastic resistance. With regard to transverse stress, a bar of Cowles's No. 2 brass, 1 inch square and 12 inches between supports, showed a breaking load of 72 cwts.; the deflection with 44 cwts. load was 04 of an inch, and the permanent set 01. Aluminium-brass forges hot, and, owing to its special casting qualities, low specific gravity, strength, toughness, and rigidity, is very suitable for ships' propellers, rudder frames, pumps, valves, pinions, etc., as well as for hydraulic work. With regard to casting, practically the same rules apply, as those given for aluminium-bronze. Its working qualities, when the amount of aluminium is high, are similar to those of the bronze. The working qualities are also governed largely by the quantity of zinc contained in the brass-the higher the percentage of zinc, the harder the metal, and the shorter the chip which flies from the tool.

Hercules Metal.--This is a cheaper form of aluminiumbrass made by the Cowles Company. It casts well, and whilst very hard and strong, works well under the tool. It is malleable when hot, and suitable for all castings where a strong metal is required. Its tensile strength varies from 30 to 40 tons per square inch; with an elongation of 10 to 30 per cent, according to the proportions of the constituents. It contains a proportion of iron as high as 7 per cent, together with a larger proportion of spelter than in the ordinary brass.

§ 81. The following tests were made by order of the navy department of the United States, and the table shows the favourable results of aluminium bronze and brass as compared with gun-metal.1

1 Paper by Mr. Dagger, Brit. Assoc. 1889.

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TESTS OF ALUMINIUM BRONZE AND BRASS

(UNITED STATES NAVY DEPARTMENT TESTS)

Approximate Composition.

Area in

No.

reference in inches.

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marks in

square

square inch.

15 inches.

Reduction of area per cent.

Diameter at fracture in inches.

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TESTS OF HARDNESS OF LARGE BARS. HARDNESS OF NAVY YARD BRONZE. (Aluminium Bronze, or Brass.)

The Navy Yard Bronze was furnished by the United States Government, and carefully cast at New York Navy Yard, under inspection of an officer detailed for that purpose.

WILLIAM H. HARRIS,

Chief Engineer, U.S.N.

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