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What is the composition of following alloys ? What are their special characteristics and what for these are used ?

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(i) Permalloy, (ii) Invar, (iii) Elinvar, (iv) Platinite, (v) Dumet wire. Ans. (i) Permalloy has 78.5% nickel. It has a high permeability and low hysteresis loss.

(i) Invar contains 36% nickel and has an exceedingly low coefficient of linear expansion. Since its expansion is proportional to the temperature within limits of atmospheric temperature change, it is used for secondary standards of length.

(ii) Elinvar contains 32% nickel with small percentage of Cr, W, Mn, Si and C. In addition to a low coefficient of expansion, it also has a constant modulus of elasticity over the temperature range of 0 to 38°C and is thus used for hair springs for watches and springs for other precision instruments.

(iv) Platinite contains 46% nickel. It has the same thermal coefficient of expansion as platinum and thus is used as a substitute for platinum.

(v) Dumet wire contains 42% nickel. It is covered with copper to prevent degassing at the seal and is used to replace platinum as the "seal in" wire in incandescent lamps and vacuum tubes.

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