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Table showing the comparative th« Total resistance of circuit in every case about 2048 x 10" absolute— nearly proportional to the strengths of currents.
Heat,' that if the condition of metal at a certain temperature depended exclusively on that temperature, no distribution or movement of heat could possibly give rise to a current of electricity in a circuit of one metal; nevertheless I find, as above* stated, that in a circuit of one metal wire a current is maintained for five minutes at a time, gradually vanishing to nothing when the two ends of the homogeneous wire have been for some time in contact, but recommencing if one wire is cooled for a minute and then again applied to the hot one. One explanation of this might be that the condition of the wires does not solely depend on their temperature, but is influenced to a considerable extent by the time during which they have remained at that temperature. Nor is this a gratuitous assumption: Dr. Matthiessen has proved that wires of several metals do not attain a constant conducting power until they have been kept for some time at a constant temperature; he finds that the conducting power of bismuth increases, whilo that of tellurium decreases when kept for a time at 100°. Quito similarly, somo metals may rise and some may fall in the thermo-electric scale after being heated for some time, a supposition which is necessary to account for the metallic contact currents by the theory I suggest.
Another possible explanation of the metallic contact currents may he found in a partial hardening on tho one side and annealing on the other, caused by the sudden contact of the hot and cold metal. If this bo so, the current between annoaled and unannealcd wires of the same metal would correspond with the contact current between two homogoncous wires, in a way which it does not seem to do.
I am, however, now engaged in investigating this subject, and hope before next year to be ablo to give facts which may decide whether either of these theories is tenable There is groat difficulty in forming any conclusion from experiments hitherto made, inasmuch as none of tho observers, except Dr. Matthiessen, have used chemically pure metal, and it is found that the electrical properties of a metal aro affected to an extraordinary degreo by tho presence of impurities in very small quantities.
Explanation of the Table.
The names of the metals of which tho loops were made aro entered at the side and top of the Table. The experiments made with each combination aro entered in the subdivision at the intersection of tho horizontal and vertical columns corresponding to tho two metals. Tho metals named at the top formed tho right-hand loop, those at the side tho left-hand loop. The arrows show the direction of the current across the joint. Tho first entry in each subdivision shows the deflection observed when the right-hand metal was heated and the wires held loosely together. The second entry shows tho deflection when tho samo metal was heated but tho wires drawn tightly together.
Tho third entry gives tho maximum deflection, and the direction of the current, when the middle of tho joint is gradually heated and tho two wires held tightly together.
The fourth entry (where given) shows tho maximum deflection from a current in the opposite direction when greater heat was applied. The two last entries show the common well-known metallic thermo-electric effects. Tho first entry shows the new loose-contact effect. The second entry shows an uncertain combined effect of metallic and imperfect contact effects.
An example will perhaps make this clearer. "When copper and iron wore 1862. N
used and copper loop heated, a loose contact produced a current from copper to iron across the joint, giving a deflection of 100 divisions. A tight contact gave nothing decided. "When the iron loop was heated (the copper cold) the loose contact produced a current from iron to coppcr«across the joint, giving a deflection of 90 divisions. A tight contact in this case gave a weak current in the opposite direction. When the joint was heated in the middle, as the temperature gradually rose, a maximum deflection of 3 divisions was first reached, showing a current from copper to iron across tho joint; and as the heat increased still further this current was reversed, and finally, at a white heat, gave a maximum deflection of 3 divisions with a current from iron to copper.
On the Mechanical Properties of Iron Projectiles at High Velocities. By W. Fairbairn, F.R.S.
A Valuable series of experiments were made at Manchester upon portions of plates fired at by the Iron Plate Committee at Shoeburyness. These experiments comprised the determination of tho resistance to punching, to a tensile strain, to impact, and to pressure.
They show that the tenacity varied from 11 to 29 tons per square inch in the iron plates, and from 26 to 33£ tons in the homogeneous iron plates. The average strength of the iron plates between 1| and 3 inches thick varied from 23£ to 24£ tons per square inch, and this, or about 21 tons, may probably be insisted upon as a measure of strength in future contracts for iron plates.
The elongation of the plates under a tensile strain may be taken as a measuro of the ductility of the material; it varied in the thicker iron plates from 0-91 to 0-27 per unit of length, and averaged 0-27 inch in tho homogeneous metal plates. The maximum observod was 0-35.
The most important results in connexion with the question of the resistance aro, however, those obtained by combining tho tensile breaking weight with the ultimate elongation, as first indicated by Mr. Mallet in a paper read before the Institution of Civil Engineers. By finding in this manner the product of the tenacity and ductility, numbers are obtained which, though not identical with thoso expressing tho resistance of the plates in the experiments with guns at Shoeburyness, are yet in close correspondence with them. The average value for Mr. Mallet's coefficient in the thicker iron plates was about 6500 lbs., and in the steel or homogeneous plates 8300 lbs. But the resistance of tho iron plates increases with tho thickness, whilst that of the homogeneous metal diminishes. The correspondence of these numbers is indicated in the Report addressed to the War Office and the Admiralty; but a more extended series of experiments are yet wanting to determine the true value of tho coefficient as a guide to bo insisted upon in the manufacture of iron plates. 9000 foot-pounds is the maximum for iron given by the results already obtained; but an extended series of experiments might develope new features of resistance and new improvements in tho manufacture.
The experiments on punching afford an explanation of tho greatly increased perforating power of the flat-headed shot over that of the round-headed projectiles. They also lead to a formula for the ordinary cast-iron service shot, which appears to give with approximate accuracy the law of the resistance of plates of different thicknesses to missiles of various weights and velocities.
These investigations led to inquiries into the state of the manufacture of plates calculated to resist heavy and powerful projectiles directed against the sides of an iron-plated ship, and, moreover, to determine the exact thickness of plates that a vessel was able to carry. Again, they had reference to the quality of the plates and their powers of resistance to impact. There were three conditions necessary to be observed in the manufacture: 1st, that the material should bo soft and ductile; 2nd, that it should be of great tenacity; and, lastly, that it should be fibrous and tough. All these conditions apply to the manufacture of plates, and they also apply, with equal force, to the projectiles in their resistance to pressure and impact.
In the experiments at Shoeburyness, it was found that the ordinary castiron service shot were not adapted for penetration, as they invariably broke into fragments when discharged against a sufficiently thick armour-plate. In most cases when delivered at high velocities, they had the power of damaging and breaking the plates; but owing to their crystalline character and defective tenacity, a considerable portion of the power was expended in their own destruction. To some extent the same law was applicable to wrought-iron shot, as part of the force, from its greater ductility, was employed in distorting its form, and depriving it of its powers to penetrate the plate. Cast and wrought iron are therefore inferior as a material for projectiles intended to be employed against iron-plated ships and forts. With steel hardened at the end the case is widely different, as its tenacity is not only much greater than that of cast and wrought iron, but the process of hardening the head prevents compression and its breaking up by the blow when the whole of its force is delivered upon the plate. Steel, although much superior to cast or wrought iron in its power of resistance in the shape of shot, is, nevertheless, susceptible of distortion and compression, and in every instance when employed against powerful resisting targets the compression, and consequently the distortion, was distinctly visible.
There is another consideration besides the material which enters largely into the question of the resisting powers of shot, and that is form. It will be recollected that, some years since, the late Professor Hodgkinson instituted a series of experiments to determine the strength of iron pillars, and the results obtained were in the following ratios:—
1st. That pillars of about 20 to 30 diameters in length, with 1 Qqqq
two flat ends, broke with J
2nd. Pillars with one end rounded and one flat broke with 2000
being in the ratio of 1, 2, 3. Now in order to ascertain the effects of form on cylindrical shot, a series of experiments were instituted to determine the force of impact and statical pressure produced upon shot of different shapes, and from these experiments the following results were obtained.
The description of shot experimented upon was cast-iron of the cylindrical form, with flat and round ends; and it is interesting to observe that the results correspond with those where both ends are rounded and one end only rounded, as obtained by Mr. Hodgkinson on long columns; but in the short specimens with both ends rounded the results arc widely different, as may be seen by the following Table.