On Laboratory Arts

by Richard Threlfall

After the arrow is shot, it remains to gather in the thread, and if the latter is at all thin, we have a rather troublesome job. In a thread thirty or forty feet long, the most uniform part generally lies in the middle if the thread is thin, i.e. of the order of a ten-thousandth of an inch in diameter. If the thread is thick the most uniform part may be anywhere. The part of the thread required is generally best isolated by passing a slip of paper under it at each end and cementing the thread to the paper by means of a little paraffin or soft wax, and then cutting off the outer portions. One bit of paper may then be lifted off the calico, and the thread will carry the other bit. In this way the thread may be taken to a blackened board, where it may be mounted for stock.

By passing the two ends of the thread under a microscope, or rather by breaking bits off the two ends and examining them together, it is easy to form an Opinion as to uniformity.

Mr. Boys has employed an optical method of examining threads, but the writer has invariably found a high-power microscope more convenient and capable of giving more exact information as to the diameter of the threads.

The beginner — or indeed the practised hand — need not expect to get a thread of the exact dimensions required at the first shot. A little experience is necessary to enable one to judge of the right thickness of the needle for a thread of given diameter. The threads are so easily shot, however, that a few trials take up very little time and generally afford quite sufficient experience to enable a thread of any required diameter to be prepared.

It is no use attempting to heat an appreciable length of needle; if this be done the thread almost invariably has a thick part about the middle of its length.. It is sufficient to fuse at most about one-twentieth of an inch along the needle before firing off the bow. This can be done by means of the smaller oxygas blow-pipe jet described in the article on blow-pipes for GLASS-BLOWING, § 14. The flame must of course be turned down so as to be of a suitable size. A sufficiently small flame may be got from almost any jet.

If the needle be not equally heated all round, the thread tends to be curly; indeed by means of the catapult, threads may be pulled which, when broken, tend to coil up like the balance-springs of watches, if only care be taken to have one side of the needle much hotter than the other.

§ 85. When examining bits of threads, say thicker than the two-thousandth of an inch, under the microscope it is convenient to use a film of glycerine stained with some kind of dye, in order to render the thread more sharply visible. The thread is mounted beneath a cover slip, and a drop of the stained glycerine allowed to run in. Such a treatment gives the image of the thread a sharply defined edge 3 and the contrast between the whiteness of the thread and the colour of the background allows measurements to be made with great ease.

On the whole the easiest way of measuring the diameter of a thick thread is to use a measuring microscope, i.e. one in which the lens system can be displaced along a plane bed by means of a finely cut micrometer screw. The instruments made by the Cambridge Scientific Instrument Company do fairly well. Direct measurements up to 0.0001 inch are easily made by means of a microscope provided with a Zeiss "A" objective, and rather smaller differences of thickness can be made out by it. For thin threads the method next to be described is more fitting, because higher powers can be more conveniently used.

In this method an ordinary microscope is employed together with a scale micrometer, and either an eyepiece micrometer, or a camera and subsidiary scale. The eyepiece micrometer is the more convenient. If a camera be employed, i.e. such an one as is supplied by Zeiss, it is astonishing how the accuracy of observation may be increased by attending carefully to the illumination of both the subsidiary scale and of the thread. The two images should be as far as possible of equal brightness, and for this purpose it will be found requisite to employ small screens.

The detail of making a measurement by means of the micrometer eyepiece is very simple. The thread is arranged on the stage so as to point towards the observer, and the apparent diameter is read off on the eyepiece scale. In order to calibrate the latter it is only necessary to replace the thread by the stage micrometer, and to observe the number of stage micrometer divisions occupying the space in the eyepiece micrometer formerly occupied by the thread. It is essential that both thread and stage micrometer should occupy the same position in the field, for errors due to unequal distortion may otherwise become of importance. For this reason it is best to utilise the centre of the field only.