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ART. IV. Experiments on the Tenacity of Iron Wire, with some Account of the Wire Bridges of Geneva. By COLONEL DUFOUR. [Bib. Univ.]

THE extreme economy and facility of construction of Wire Bridges, are circumstances which cannot but tend to introduce them into very general use: hence a knowledge of the strength of Iron Wire, as generally prepared by the manufacturers, and the circumstances which have an influence over it, cannot but possess interest. The following experi ments by M. Dufour, being made with a practical view, are therefore very valuable, and have already assisted in furnishing data for the construction of two Wire Bridges across the fortification ditch of Geneva.

The object of the experiments was to determine the abso lute strength of wires of different diameters; their elongation when sustaining a given weight; the effects of a sudden concussion; the influence of annealing at a red heat, and the effect of a fold, or return, or junction of the wire, in deter mining rupture when in these circumstances.

Four kinds of Iron Wire were chosen, having the respective diameters of 1, 2, 3, and 4 millimetres, nearly. Six experiments on the finest wire, of which the diameter was 0.85, mm. (0.033 of an inch,) proved that the strength was independent of the length; that the mean absolute force of such a wire was 106 lbs avoirdupois, the extremes being 103.7 and 120; and that when annealed, it sustained only 46.3 lbs. Ten experiments on the second wire, diameter 1.9 mm. or 0.748 of an inch, gave 432.5 lbs as the mean weight it could sustain, the extremes being 397 and 457; from which it would appear that the first wire had a seventh more of strength in proportion to its diameter than the second. The second wire, when annealed, sustained only 223 lbs, which is to its strength when unannealed, as 100 to 194. The third wire about .118 of an inch in diameter, sustained as the mean of six experiments, 843 lbs: when annealed its strength was to that of the unannealed wire, as 100 to 195. The fourth wire of a diameter of .145 of an inch, supported 1713 lbs when unannealed, and 889 lbs when annealed, the ratio in the two states being as 100 to

192.

From these experiments Colonel Dufour concludes that Iron Wires from 1 to 4 millimetres in diameter, support at least 132 lbs for each square millimetre of their sec

tion. But according to known experiments on forged bars of iron, it has been ascertained, that those which are not more than 6 mm. square, do not support more than from 88 to 100 lbs per square millimetre, and those which are larger only from 55 to 66 lbs, a circumstance which sufficiently proves the advantage of employing iron drawn into wires, rather than forged into bars, when the question relates to its tenacity.

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The second object of the experiments was to ascertain the elongation of a wire when submitted to a weight, less than that sufficient to break it. The elongation due to the mere rectification of the sinuosities and curves in the wire itself, was found to be of the original length, when a bundle of twelve wires of the second kind before referred to and 30 feet long, was charged with a weight of 6621 lbs. Another kind of elongation immediately precedes the rupture, and is due to a slight diminution of diameter. It may be perceived when the wire is charged with two-thirds of the weight, it is capable of supporting; and varies between 35 and 37 ten-thousandths of the length. When the wire is annealed the elongation is very considerable, and about thirteen-hundredths of the total length in all the wires tried. The influence of folds, returns, &c., on the tenacity of the wire was of great importance considering the object of the experiments; the following are some of the practical results obtained. When a wire is passed round a ring or cylinder, so as to return parallel to itself, and bear a force applied to the two extremities nearly double that supported by the single wire, it requires that the diameter of the cylinder round which it passes should be at least 14 inch. In proportion as the diameter is smaller, or the curvature of the wire greater, its tenacity diminishes, and the wire will constantly break at that place. One or more entire revolutions of the wire on the same cylinder must be avoided, because the friction resulting from such an arrangement opposes the equality of stress which is required upon each of the several wires constituting a bundle.

After many experiments on the different means of joining wires together, experience pointed out as the most efficient, one which would perhaps not have been indicated by theory. The method was to lay the ends side by side one over the other, and bind them round for the space of at least 13 inch, by a smaller annealed iron wire. Such a junction al ways resisted the proofs applied, the wires constantly giving way at some other place.

The preceding experiments were made with weights gradually accumulating, and unaccompanied by any sudden impulse or momentum, but as in their application to the construction of bridges, effects of the latter kind would be continually occurring, further experiments were made of this nature. The wires were therefore charged with about half the entire weight they were able to support, and then other weights were dropped from different heights into the box containing the previous charge. The latter force was always estimated by its momentum, and experiment proved that the second wire, for instance, charged with half the weight it was just able to bear, could sustain without risk a quantity of momentum equivalent to 3000, the weight being given in killograms (2.207 lbs) and the velocity by the centimetres traversed in a second.

Other experiments were made with reference to the effect of temperature on the tenacity of the wire. Some Iron Wire was procured of an inch in diameter, and the weight required to break it ascertained from the mean of several experiments. A portion was then passed through a hollow vessel filled with a frigorific mixture which lowered the temperature to -8° F. In three experiments, in which wires, thus circumstanced, were broken by weights applied to them, the separation took place out of the vessel, and the weight required was the same as before. The vessel was ther filled with boiling water, and the wire passing through it, tried as before. It broke once in the vessel, and once out of the vessel, the latter by the smaller weight. Finally, two vessels were then disposed on the wire, one containing the frigorific mixture, the other boiling water; the wire gave way between them, requiring the same weight as before.

It may thus be considered as demonstrated, that between the limits of temperature indicated, i. e. 212° and 8° F., change of temperature has no influence on the tenacity of Iron Wire.

The preceding researches have been applied with the greatest success, in the construction of two bridges across the dry ditches of the fortifications of Geneva. The first of these ditches is 33 feet deep and 108 feet wide at the site of the bridge; the second is 22 feet deep and 77 feet wide; they are separated by what is called the counterguard, which is about 70 feet wide, and the top of which is level with the surrounding soil. A stone building is erected on the city edge of the first ditch, which serves as a point of attachment for the wires, as a gate to the city, and also as a sta

tion for the persons who have charge of the bridge; a piece of masonry is erected on the counterguard, as a point of support for both bridges; and a third erection of a similar kind serves as an outer gate, and for a support to the end of the outer bridge. The wire used is of the kind called No. 14 in commerce, very nearly of the diameter of the second sort referred to in the preceding experiments; it is made up into lengths or bundles, each containing 100 wires, and there are three such collections on each side of the bridge. As the line of suspension proceeds uninterruptedly across both ditches and the intervening bank, the length was found too great for one bundle; they were therefore made in shorter lengths, terminating at each end with a ring, and were connected by placing these rings side by side, and passing a strong iron bolt through them. Each single wire was first stretched by a weight of 200 lbs, then made up into the bundles of 100 each, which were united by iron ties at successive intervals, and the whole rolled round with iron wire, which gives to them the appearance of cords. The longest of these bundles are 120 feet each, the others were made shorter, as being more convenient for the situation they would occupy in the line of suspension. From this arrangement it is evident that each of the six main lines of suspension may be considered as one bundle, though consisting of many parts; they are made fast at one extremity to a plate of iron firmly attached to the stone gate before mentioned, then pass over the first ditch, across the stone support on the counterguard, over the second ditch, over the second standard, and are finally made fast to iron bars, which being attached to plates, are loaded with masses of stone and buried in the earth.

From the six principal lines other lines descend, consisting each of twelve wires only; these are made fast to the traverses, or pieces of wood which form the bases of the bridges. On these are mortised long pieces of carpentry, which are bolted together with them, and to which are fastened the railings of the bridges, and then other planks are fastened across these again, forming the path of the bridge. The rapid and complete success of this undertaking does great honour to M. Dufour, the engineer. It was planned and executed in the short space of six months. Its expense was previously estimated at 16,000 francs, and the cost amounted to within one or two hundred francs of that sum. This accuracy of estimation is not the least merit of M. Dufour. The expectations with regard to the duration of the

bridges are all in their favour; the iron is defended from rust by a thick coat of paint, which is to be renewed when required; the wood work is of select materials, and not being any where in contact with the earth is not liable to rot. Before constructing the large bridges, a model was made 38 feet long, and having only two suspending lines, each composed of 12 wires of .073 of an inch in diameter. The foot way was constructed on 11 wooden traverses, which hung from the suspension lines each by only four single wires, two at each end. This bridge was submitted to the roughest trials on the part of those persons who were curious to examine it, such as leaping, marching, &c. but without the least accident or failure.

Speaking of the comparative strength of iron in wires and in bars M. Dufour says, "The immense advantage of employing iron in wire rather than in bars, is thus rendered evident; it is more manageable; its strength is double; the strength may be better proportioned by putting the number of wires necessary to the resistance required, and a certainty is obtained of the state of the interior parts of the suspending lines, which nothing can give when large bars are used."

"It appears at first that the minimum of the force of the wire should be calculated upon, and not the mean; but as each bundle contains many wires, although there may be some of a smaller strength, there will be others that will surpass in strength, and thus the mean should be used in estimating the strength of the whole, although in employing a single wire the minimum only ought to be taken."

With regard to the Geneva bridge, M. Dufour says, that after a period of four months in which the bridge had been in full use, it had not suffered the slightest alteration in its primitive form. The path has retained the degree of curvature given it at first, and no sensible lengthening of the wires has occurred. The bridge, however, has been well tried, curiosity has taken great numbers of persons on to it at once, and all the large stones required in the latter part of the work, were taken over it on carriages without the slightest damage. The elasticity of the bridge is also what it was at first, a man walking with a moderate step does not at all disturb the steadiness of the path; on walking quickly there are slight vibrations produced, but no oscillations, and the vibrations are such as never to be communicated from the one bridge to the other, or in any way to affect the ma

sonry.

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