[23]The lengths of circumference were taken, in this and other hoops, after each cooling, by encircling the periphery with a very fine piece of “crinoline” steel, the ends of which were made just to meet round the original hoop. By again encircling the hoop with the same piece of steel the expansion was shown by a gap between the ends, and a contraction by an overlap, either of which was measured with great accuracy by means of a finely divided scale.
[23]The lengths of circumference were taken, in this and other hoops, after each cooling, by encircling the periphery with a very fine piece of “crinoline” steel, the ends of which were made just to meet round the original hoop. By again encircling the hoop with the same piece of steel the expansion was shown by a gap between the ends, and a contraction by an overlap, either of which was measured with great accuracy by means of a finely divided scale.
“Two wrought-iron bars, 11⁄8inches square and 30.05 inches long, were selected.[24]No. 1 was of common “Crown” quality; No. 2 of a superior quality known as “Tudhoe Crown.” These bars were heated to redness in a furnace and then plunged into water of ordinary temperature, the length being accurately measured after each cooling. After fifteen heatings and coolings the permanent contraction on No. 1 bar was 2.26 per cent. of the original length, and that on No. 2 bar 1.86 per cent., or an average on the two bars of about .13 per cent. after each cooling, the increment of contraction being nearly equal after each successive operation. It is noticeable that after the first two coolings the better quality of iron did not contract quite so much as the common quality, and that in the latter the contraction was going on as vigorously at the fifteenth as at the first cooling.
[24]In some of these experiments the original sizes of the iron were only measured with an ordinary foot-rule, in which case the dimensions are given in the ordinary fraction used in expressing the mercantile sizes of iron. When accurate measurement was taken decimals are invariably used both in this paper and the Tables of Experiment.
[24]In some of these experiments the original sizes of the iron were only measured with an ordinary foot-rule, in which case the dimensions are given in the ordinary fraction used in expressing the mercantile sizes of iron. When accurate measurement was taken decimals are invariably used both in this paper and the Tables of Experiment.
“Similar bars of wrought iron, heated to redness and then allowed to cool in air at ordinary temperature, do not appear to suffer any permanent change in their length.
“Experiment No. 3 was made to verify this.
“Experiment No. 3.—Wrought-iron bar, 11⁄8inches square by 30 inches long heated to a dull red and cooled gradually in air.
Wrought iron rectangular plate. 14” thick × 11” 995 × 598 planed on both surface and edges. Heated to redness, and cooled in water 50 times. The dotted lines show original form, the black lines the form after the eperiment.(Two-ninths of full size.)Fig. 1430.
Wrought iron rectangular plate. 14” thick × 11” 995 × 598 planed on both surface and edges. Heated to redness, and cooled in water 50 times. The dotted lines show original form, the black lines the form after the eperiment.
(Two-ninths of full size.)
Fig. 1430.
The plate was subjected to fifty heatings to redness and subsequent coolings in water of ordinary temperature. At every tenth cooling accurate measurements were taken of the contraction in superficial dimensions, andFig. 1430shows the final form after fifty coolings. The intermediate measurements at every tenth cooling showed a uniform and gradual decrease in the superficial dimensions, but the thicknesses were only measured after the fifty coolings had been completed. The thickness appears to have varied considerably; in some places, notably towards the centre and outside edges, being much reduced. Between the centre and outside edges the thickness appears to have increased, and in some few places the plate has been split open. The average dimensions in inches before and after the experiment were as follows (dimensions of cracks being allowedfor):—
“Three triangular pieces of iron were then cut out of the plate from positions indicated on the diagram; No. 1afrom the part most reduced in thickness, No. 3afrom the part most increased in thickness, and No. 2afrom a part where the thickness was a mean between the thickest and thinnest part. The specific gravities were accurately determined asfollows:—
“The average of these specific gravities is 7.562.
“The average before experiment was 7.64. Hence the average loss in specific gravity has been 1.02 per cent.
“The small triangular piece No. 1a, specific gravity 7.552 (already subjected to fifty heatings when forming part of the solid plate), was next heated and cooled fifty times more. The specific gravity at the end of the one hundred total coolings was 7.52, being .43 per cent. lower than after fifty heatings in plate, and 1.57 per cent. lower than 7.64, the original mean specific gravity of the plate.
“The same piece, 1a, was then heated twenty-five times more, making 125 in all. On taking the specific gravity it was found to be 7.526, or practically the same as after 100 total heatings and coolings.
“It thus appears that there is an undoubted decrease in specific gravity on repeated heating and cooling as described up to one hundred coolings, the specific gravity decreasing as much as 1.57 per cent.; that this percentage appears to be less when the pieces of iron operated upon are very small; that while there is a decrease of specific gravity there is also a decrease of total volume.
“From the above it was evident that the volume was affected by severalcauses:—
“1. By the permanent contraction of the outer skin, either the volume would be lessened, or relief by bulging out the sides must occur.
“2. By the decrease of specific gravity an increase of volume must occur, which could also find relief in bulging.
“3. A diminution of the whole mass must occur through scaling of the surface.
“Having determined the change in specific gravity by Experiment 6, we only now want to determine the loss of volume due to surface scaling, and we can then infer the actual contraction of the outer skin.
Fig. 1431Fig. 1431.
Fig. 1431.
“Experiment No. 7.—To ascertain the amount of scaling which took place in heating and cooling under same conditions as Experiment No. 6, a wrought-iron plate was cut from the same piece as No. 6, thickness .74 in., planed on both surfaces and all edges to a form nearly rectangular, and to the dimensions given inFig. 1431.
“The only difference (except the very small difference in the dimensions) between this and1430, was that the principal grain of the iron was in1431in the direction of the arrow, whereas in the other it was lengthwise of the plate.
“This piece was subjected to fifty heatings to redness and sudden coolings in water of ordinary temperature, as in the case of No. 6. The change in form was exactly the same in general character, but the contraction was not quite so great either in length or breadth; the increase in thickness, however, was proportionately greater, the volume (measured by displacement of water) after fifty heatings being 48.6 cubic inches, which is nearly the same as in No. 6 after the same number of heatings. The weight of thepiece:—
“This represents a loss of 9.07 per cent. of the original weight by scaling, and upon the whole original surface (sides and edges) represents a thickness of .0284 of an inch for the fifty immersions, or .00057 of an inch for the thickness of the film lost at each immersion over the whole surface.
“Calculating the weight of No. 6 before and after experiment from the volumes and specific gravities, we find thefollowing:—
the ascertained difference in the case of No. 7 being 1.332, thus sufficiently accounting for the discrepancy between specific gravity and change of volume by the scaling.
“By Experiment 7 it has been shown that the loss of thickness due to scaling after fifty immersions was .0284 inch over the whole surface (sides and edges.) Therefore, assuming this scaling as uniform over the surface, the girth, whether measured lengthwise or breadthwise, should be eight times .0284, or .23 inch less after immersion than before. Now the gross loss of girthis:—
“Comparing these results with those of Experiments Nos. 1, 2, and 5, we find that the contraction of the skin of the plate is less for each immersion than that of a bar or hoop, in the proportion of .125 to .083. This is what might be expected, as the contraction of the plate is resisted by the volume of heated matter inside, which is eventually displaced by bulging, while the bar finds relief endwise without having to displace the interior.
“We have now before us the following facts, substantiated by the experimentsdescribed:—
“1. That in heating to redness, and then cooling suddenly in water at ordinary temperatures, bars and plates of wrought iron, a reduction of specific gravity takes place, the amount being about 1 per cent. after fifty immersions, and 1.57 per cent. after one hundred immersions, further heatings and coolings not appearing to produce further change.
“2. That a reduction of the surface takes place after each heating and cooling, this being due to twocauses:—
“a.The scaling of the surface, which is shown to amount to a film over the (sides and edges) entire area of .00057 inch in thickness for each immersion, or 0.284 inch for fifty immersions (Experiment 7).
“b.A persistent contraction, which takes place after each immersion. This varies according to the form of the iron, being in plates from .07 per cent. to 0.83 per cent. (Experiment 6), while in long bars it varies from .122 to .15 per cent. (Experiments 1, 2, and 5). This contraction continues vigorously up to fifty immersions, and probably much farther.
“3. That in the case of plates a bulging takes place on the largest surfaces, increasing the thickness towards the centres, although the edges diminish in thickness.
“4. That wrought-iron bars heated to redness, and allowed to cool slowly in air, do not show any change in dimensions (Experiment 3).
“The reduction of specific gravity, and the bulging out of the sides, have been explained as follows by the learned Secretary of the Royal Society, Professor Stokes, who has taken considerable interest in these experiments, and who has kindly allowed the author to publish the explanation:
“‘When the heated iron is plunged into water, the skin tends everywhere to contract. It cannot, however, do so to any significant extent by a contraction which would leave it similar to itself, because that would imply a squeezing in of the interior metal, which is still expanded by heat, and is almost incompressible. The endeavor, then, of the skin to contract is best satisfied, consistently with the retention of volume of the interior, by a contraction of the skin in the two longish lateral directions, combined with a bulging out in the short direction. The still plastic state of the interior permits of this change.
“‘Conceive an india-rubber skin of the form of the plate in its first state, the skin being free from tension, and having its interior filled with water, treacle, or pitch. I make abstraction of gravity. It would retain its shape. But suppose, now, the india-rubber to be endowed with a tension the same everywhere similar to that of india-rubber that has been pulled out, what would take place? Why, the flat faces of considerable area, being comparatively weak to resist the interior pressure, would be bulged out, and the vessel would contract considerably in the long directions, increasing in thickness. This is just what takes place with the iron in the first instance. But when the cooling has made further progress, and the solidified skin has become comparatively thick and strong, the further cooling of the interior tends to make it contract. But this it cannot well do, being encased in a strong hide, and accordingly the interior tends to be left in a porous condition.’
“The reduction by scaling does not require any explanation. The only fact which appears unaccounted for is this persistent contraction of the cooled iron skin, which does not appear to be explicable on any mechanical grounds; and we are, therefore, obliged to look upon it as the result of a change in the distance of the molecules of the iron, caused by the sudden change of temperature in the successive coolings.
“Our next subject is the curious effect of cooling bars or rings by partial immersion in water. Bearing in mind the results at which we have arrived, viz., that wrought iron contracts when immersed in water after heating, and that when allowed to cool in air it remains of the same dimensions, let us ask what would be the behavior of a bar or circular hoop of iron cooled half in water and half in air, the surface of the water being parallel to the fibre and at right angles to the axis of the hoop?
“Arguing from the results of Experiments 1, 2, and 5, it might be expected that the lower portion cooled in water would suffer permanent contraction; and, arguing from Experiment 3, that the upper or air-cooled edge would not alter. This apparently legitimate conclusion is completely disproved by experiments. This will be seen by a reference to Experiments 8, 9, and 10.
Fig. 1432Fig. 1432.—Experiments with a circular hoop of wrought iron. Appearance of the hoop at the beginning.
Fig. 1432.—Experiments with a circular hoop of wrought iron. Appearance of the hoop at the beginning.
Fig. 1433Fig. 1433.—Condition of the hoop after the twentieth cooling.
Fig. 1433.—Condition of the hoop after the twentieth cooling.
“In No. 8 a circular hoop of wrought iron was forged out of a 31⁄2-inch by1⁄2-inch bar, the external diameter being about 18 inches, the breadth,1⁄2inch, being parallel to the axis of the hoop. This hoop,Fig. 1432, was heated to redness, then plunged into cold water half its depth, the upper half cooling in air. The changes in the external circumference of the hoop were accurately measured after each of twenty successive coolings, at the end of which the external circumference of the water-cooled edge had increased 1.24 inches, or 2.14 per cent. of its original length, and the air-cooled edge had contracted 7.9 inches, or 13.65 per cent.
“Experiment No. 8.—Wrought-iron hoop, 31⁄2inches by1⁄2inch by about 18 inches in diameter, or exactly 57.85 inches in circumference at top, and 57.95 inches at bottom edge.
“It will be observed that we have here two remarkable phenomena: 1. The reversal of the expansion and contraction as described. 2. The very large amount of contraction on the upper edge compared with what was exhibited in Experiment 5 of entire submersion.
“The table showing Experiment 5 gives a contraction of 2.25 per cent. after the twentieth cooling, whereas the contraction on the air-cooled edge of Experiment 8 is 13.65 per cent., or six times the contraction of an entirely submerged hoop.
“To ascertain whether these unexpected phenomena had any connection with the circular form of the hoop, Experiment 9 was made with a straight bar of iron 31⁄2inches deep by1⁄2inch thick by 28.4 inches long.
“Experiment No. 9.—Wrought-iron bar, 31⁄2inches by1⁄2inch by 28.4 inches long, heated to a dull red, then quenched half its depth in water.
“This was cooled half in air and half in water, and the length of the two edges measured accurately after each of twelve coolings. At the end of this experiment the air-cooled edge had contracted 6.9 per cent., while the water-cooled edge had expanded 1.55 per cent. of the original length. The effect on thebar was to make it gradually curve, the water-cooled or extended edge becoming convex, the air-cooled or contracted edge concave.
Fig. 1434Fig. 1434.—Experiments with a wrought-iron bar. Appearance of the piece before heating.
Fig. 1434.—Experiments with a wrought-iron bar. Appearance of the piece before heating.
Fig. 1435Fig. 1435.—Appearance of the bar after the twelfth cooling.
Fig. 1435.—Appearance of the bar after the twelfth cooling.
Fig. 1436Fig. 1436.—After the preceding experiment the same bar was reheated and reversed in the water, the eleventh cooling resulting in the above form, the bar bending in the opposite direction from that previously shown.
Fig. 1436.—After the preceding experiment the same bar was reheated and reversed in the water, the eleventh cooling resulting in the above form, the bar bending in the opposite direction from that previously shown.
“Experiment No. 10 was made in order to show the effect of reversing this cooling process. After five coolings, a bar of iron, 28 inches long, 31⁄2inches deep, and1⁄2inch thick, was curved so that the versed sine of its air-cooled edge was 11⁄2inches. The coolings were then reversed, what was the air-cooled edge being then immersed in water. After five more coolings the bar was restored to within1⁄8inch of being straight, and the eleventh cooling threw the concavity on the other side of the bar.
“Experiment No. 10.—Wrought-iron flat bar, 28 inches long by 31⁄2inches by1⁄2inch, heated to dull red, then quenched half its depth in water, up to five heats, then the opposite edge dipped.
“When the author had proceeded thus far, these curious results were shown to several leading scientific men, who expressed interest in the subject, which encouraged the author to extend his experiments under varied conditions with a view of ascertaining the cause for these anomalous effects. These experiments (Nos. 11 to 17) are fully recorded, and the results shown on the diagrams; the actual rings are also on the table before you.
Largeimage(49 kB).Fig. 1437Fig. 1437.
Largeimage(49 kB).
Fig. 1437.
“Experiment No. 11.—Wrought-iron hoop, turned and bored, 37.1 inches, outside circumference, by 2.95 inches deep by .44 inch thick, the grain of the iron running the short way of the bar from which the hoop was made, heated to redness, then cooled half its depth in water (seeFig. 1437atafor final form of hoop after ten heatings and coolings).
“Experiment No. 12.—Wrought-iron hoop, turned and bored, 6 inches diameter (18.85 inches circumference) outside, by 2 inches deep by .375 inch thick, heated to redness, then cooled, with lower edge barely touching the water (seeFig. 1437atbfor final form of hoop after twenty heatings and coolings).
“Experiment No. 13.—Wrought-iron hoop, turned and bored, 6 inches diameter (18.85 inches circumference) outside by 2 inches deep by .375 inch thick, heated to redness, then cooled one-fourth its depth in water (seeFig. 1437atcfor final form of hoop after twenty heatings and coolings).
“Experiment No. 14.—Wrought-iron hoop, turned and bored. 6 inches diameter (18.85 inches circumference) outside by 2 inches deep by .375 inch thick, heated to redness, then cooled one-half its depth in water (seeFig. 1437atdfor final form of hoop after twenty heatings and coolings).
“Experiment No. 15.—Wrought-iron hoop turned and bored, 6 inches in diameter (18.85 inches circumference) outside by 2 inches deep by .375 inch thick, heated to redness, then cooled three-fourths its depth in water (seeFig. 1437atefor final form of hoop after twenty heatings and coolings).
“Experiment No. 16.—Cast-copper ring, turned and bored to same dimensions as Nos. 12, 13, 14, and 15, heated to redness, then cooled half its depth in water (seeFig. 1437atffor final form of hoop after twenty heatings and coolings).
“It will be unnecessary to occupy much time in analyzing the experiments, as any one who takes a practical interest in the subject will have full information in the diagrams and tables. Professor Stokes drew attention to the fact that, in 1863, similar phenomena had been noticed by Colonel Clark, of the Royal Engineers. His experiments, made at the Royal Arsenal, Woolwich, were published in the ‘Proceedings of the Royal Society,’ and Professor Stokes had himself attached an explanatory note, the outline of which was asfollows:—
“Imagine a cylinder divided into two parts by a horizontal plane at the water-line, and in this state immersed after heating. The under part, being in contact with water, would rapidly cool and contract, while the upper part would cool but slowly. Consequently by the time the under part had pretty well cooled, the upper part would be left jutting out; but when both parts had cooled their diameters would again agree. Now in the actual experiments the independent motion of the two parts is impossible on account of the continuity of the metal; the under part tends to pull in the upper, and the upper to pull out the under. In this contest the cooler metal, being the stronger, prevails, and so the upper part gets pulled in a little above the water-line while still hot. But it has still to contract in cooling, and this it will do to the full extent due to its temperature, except in so far as it may be prevented by its connection with the rest. Hence, on the whole, the effect of this cause is to leave a permanent contraction a little above the water-line, and it is easy to see that the contraction must be so much nearer to the water-line as the thickness of the metal is less, the other dimensions of the hollow cylinder and the nature of the metal being given. When the hollow cylinder is very short, so as to be reduced to a mere hoop, the same cause operates, but there is not room for more than a general inclination of the surface, leaving the hoop bevelled.
“The expansion of the bottom edge was not noticed in Colonel Clark’s paper, perhaps owing to the much smaller hoops which he used in experimenting. Accepting Professor Stokes’ explanation of the top contraction, it appears that expansion of the bottom may be accounted for by the reacting strain put on the cooled edge when forcing in the top edge, acting in such a way as to prevent the cooled edge coming quite to its natural contraction, and this, when sufficiently great, expresses itself in the form of a slight expansion.
“Experiment No. 14.—Forged steel hoop, turned and bored, 18.53 inches in circumference outside by 2.375 inches deep by .27 inch thick, heated to redness, then cooled one-half its depth in water (seeFig. 1437atgfor final form of hoop after three heatings and coolings).
The shrinkage of iron and steel by cooling rapidly is sometimes taken advantage of by workmen to refit work, the principles involved in the process being asfollows:—