The elastic limit of curved wire has never been determined, since any stress that will cause it to reach a straight line is beyond the elastic limit of the inside of its sectional area.  That of curved iron wire has been estimated at 40 per cent. of its ultimate strength, which is about half the ultimate strength of curved steel wire; that is, it would be unsafe to put more than 40 per cent. of 500 lb.—­or 200 lb.—­upon a curved iron wire when a straight steel one can sustain 720 lb. without injury.  In the New York and Brooklyn bridge the cost of a sufficient amount of such iron wire as is used in all other suspension bridges would have been some $200,000 greater than that of the straight steel wire which was used.  At five per cent., this effects an annual saving in interest of $10,000.

There must, too, be a considerable saving in the current expense for painting and care, to say nothing of the more neat and elegant appearance of the less bulky steel.  And as the whole area of the section of these wires is subjected to an even strain that is always far within the elastic limit, there is no danger of a change of structure under that stress.

It is highly probable—­although Col.  Paine has been too busy to work up the matter—­that piano wire made in this straight method could be drawn up to and kept at pitch, without approaching very near the elastic limit.  In that case not only would they seldom if ever require tuning, but probably all along the tone would be more satisfactory.  And there would not be those exasperating periods when the pitch is not quite perfect, but yet is not far enough out to make it seem worth while to send for a tuner.

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