. Transactions. shape for a test specimen. Theparticular cubes used with Blacks dynamometer are so small that theaccidental errors arising from unavoidable irregularities in shape, size,and internal structure may easily render the results uncertain. Theirsmallness introduces an error owing to the restraining action of the jawsbetween which they are crushed. Temperature affects the stiffnessof the spring used for measuring the crushing force; it also affects con-siderably the strength of a dental amalgam. The strength of an amalgam Dental Cosmos (1895) 37, 406 ff. 660 METALLOGRAPHIC PHENOMENA O
. Transactions. shape for a test specimen. Theparticular cubes used with Blacks dynamometer are so small that theaccidental errors arising from unavoidable irregularities in shape, size,and internal structure may easily render the results uncertain. Theirsmallness introduces an error owing to the restraining action of the jawsbetween which they are crushed. Temperature affects the stiffnessof the spring used for measuring the crushing force; it also affects con-siderably the strength of a dental amalgam. The strength of an amalgam Dental Cosmos (1895) 37, 406 ff. 660 METALLOGRAPHIC PHENOMENA OBSERVED IN AMALGAMS specimen is also largely dependent upon the pressure under which it ismolded into form; and hand or mallet packing cannot control this pressuresufficiently well for accurate comparisons of different alloys. Standardized Procedure for Crushing-strength Tests I have, therefore,, discarded the old Black dynamometer in favor of a9000-kg. Olsen testing machine, which was designed for operation by. Fig. 3.—The 9000-kg. Olsex machine adapted to testing strength of MATERIALS AT ANY TEMPERATURE AND PROVIDED WITH POWER DRRT; FOR APPLYINGTESTING LOAD WITH REGULARITY. hand (Fig. 3). This I modified by the addition of an electric motor drivewhich applied the load with regularity, thus permitting the beam to bekept balanced to a nicety right up to the moment of failure; and by theaddition of electric heaters for bringing the specimen under test to thedesired temperature, which was determined by a suitably placed thermo-element. Additional thermo-elements enabled temperature gradientswithin the heated region surrounding the cylinder to be adjusted toneghgible values. The electromotive forces were measured by apotentiometer. The test specimens were in the form of right circular cyhnders ARTHUR W. GRAY 661 mm. in diameter and of sufficient height to ehminate errors caused bythe restraining action of the crushing jaws (Fig. 4). These cyHnderswere prepared by molding t
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Keywords: ., bo, bookcentury1800, bookdecade1870, booksubjectmineralindustries
