« VorigeDoorgaan »
For the purpose of removing the coating of silver that surrounds it, the wire must be steeped for a few minutes in warm nitrous acid, which dissolves the silver without danger of doing any injury to the gold. And though it might be difficult in this manner to preserve any considerable length of such wires, it is of little importance for any of those uses to which it is likely to be applied.
In my endeavours to make slender gold wires by the method above described, the difficulty of drilling the central hole in a metal so tough as fine silver, was greater than I had expected, and I was induced to try whether platina might not be substituted for the gold, as in that case its infusibility would allow me to coat it with silver without the necessity of drilling
Having formed a cylindrical mould of an inch in diameter, I fixed in the centre of it a platina wire previously drawn to the too of an inch, and then filled the mould with silver. When this rod was drawn to jo, my platina was reduced to Toco, and by successive reduction I obtained wires of 7000 and 706, each excellent for applying to the eye-pieces of astronomical instruments, and perhaps as fine as can be useful for such purposes.*
Since this had been the primary object that I had in view, I should have thought my time ill bestowed in pursuing farther the practical application of a method to which there seems no limit, except the imperfections of the metal employed. But as I found by trial the tenacity of these wires to be greater than was to be expected in proportion to their substance, that
• No very accurate observations can be made with a telescope shorter than thirty inches, and at thut distance asbo of an inch subtends only one second of a degree.
circumstance excited some doubts regarding the correctness of the estimate by which their diameter had been deduced. Other wires were consequently drawn with the utmost care, as to the quality and substance of the platina employed, and as to the proportional reduction of its diameter in the process of wire-drawing.
The extremity of a platina wire having been fused * into a globule nearly 1 of an inch in diameter, was next hammered out into a square rod, and then drawn again into a wire 7'55 of an inch in diameter. One inch of this wire duly coated with silver was drawn till its length was extended to 182 inches, consequently the proportional diminution of the diameter of the platina will be expressed by the square root of 182, so that its measure had become
3425 specific gravity of the coated wire was assumed to be 10,5, and since the weight of 100 inches was 114 grains, its diameter was inferred to be of an inch, or just eighty times that
42,8 of the platina contained in it.
With portions of the platina wire thus obtained, and successively reduced in diameter, I had an opportunity of repeating the trials of its tenacity with greater confidence in the justness of the estimate, and the results shewed generally (though with some accidental exceptions) that the process of wiredrawing, which is well known to improve the strength of
253 X 13,5
• I am indebted to my friend Dr. MARCET for the simple and easy method by which the fusion was effected. A piece of wire, about six inches long, having been bent to an angle in the middle, one half of its length was held in the Aame of a spirit lamp impelled by a current of oxygen, and its extremity was thus fused in about half a minute.
metals within moderate limits, continued also to add something to the tenacity of platina even as far as of an inch, which supported if grain before it broke; but the wire on which these experiments were made began then to be impaired by repetition of the operation: so that although I afterwards obtained portions of it, as small as of an inch in diameter,
30,000 it was in many places interrupted, and I could place no reliance upon any trials of its tenacity.
There are some little circumstances in the management of these fine wires, which it may be of advantage to describe for the assistance of those who would apply them to any useful purpose. When the diameter is not less than 706 or do of an inch, the difficulty of seeing and applying them in short pieces is not considerable; but when their diameter is farther reduced, and their length as much as an inch or more, the slightest current of air is sufficient to defeat all attempts to lay hold of an object so difficult to see, and so impossible to feel. It is therefore necessary to retain a part of the silver coating at each extremity, which, at the same time that it assists in finding the end, also serves to stretch the wire with a certain moderate tension, and affords the means of attaching it in any required position.
The method that I have found most convenient is to bend a portion of the coated wire into the shape of the letter U, with small hooks at its upper extremities. In this form it will conveniently hang upon a wire of gold or of platina, with the lowest part immersed in nitrous acid, till the coating of silver is removed from that part. It may then, without difficulty, be
lifted from its place, by one of the hooks alone, to any other situation, or suspended by it, with the other hook downwards, as the means of attaching a small chain, or other series of equal weights in trials of its tenacity.
XVI. Description of a single-lens Micrometer. By William Hyde
Wollaston, M. D. Sec. R. S.
Read February 25, 1813.
Having had occasion to measure some very small wires with a greater degree of accuracy than I was enabled to do by any instrument hitherto made use of for such purposes, I was led to contrive other means that might more effectually answer the end proposed. The instrument to which I had recourse is furnished with a single lens of about ts of an inch focal length. The aperture of such a lens is necessarily small, so that when it is mounted in a plate of brass, a small perforation can be made by the side of it in the brass as near to its centre as is of an inch.
When a lens thus mounted is placed before the eye for the purpose of examining any small object, the pupil is of sufficient magnitude for seeing distant objects at the same time through the adjacent perforation, so that the apparent dimensions of the magnified image might be compared with a scale of inches, feet, or yards, according to the distance at which it might be convenient to place it. A scale of smaller dimensions attached to the instrument will, however, be found preferable on account of the steadiness with which the comparison may be made; and it may be seen with sufficient distinctness by the naked without
effort of nice adaptation, by reason of the smallness of the hole through which it is viewed.