October i6, 1890] 



NATURE 



60: 



or a push, which may also be measnred by the twist of a wire if 

 it is applied to the end of a lever or arm carried by the wire. 



Now supposing that the force — whether of the nature of a 

 twist or of a pull, it does not matter which — is too small to 

 produce an appreciable twist in the wire, it is obvious that a 

 finer wire must be employed, but it is not obvious how much 

 more easily a fine wire is twisted than a coarse one. If the fine 

 wire is one-tenth of the diameter of the coarse one, we must 

 multiply ten by itself four times over in order to find how 

 much more easily twisted it is, and thus obtain the enormous 

 number 10,000 ; it is 10,000 times more easily twisted than the 

 coarse one. Thus there is an enormous advantage in increasing 

 the minuteness of the wire by means of which feeble twisting 

 or pulling forces are measured. But if the delicacy of the 

 research is such that even the finest wire which can be made 

 is still too stiff, then, even though with such wire, which is 

 somewhere about the thousandth of an inch in diameter, forces 

 as small as the millionth part of the weight of a single grain can 

 be detected with certainly, the wire is of no use ; and as wire 

 cannot be made finer, some other material must be used. Spun 

 glass is fine and strong, and is still more easily twisted than 

 the finest wire, but it possesses a property somewhat analogous 

 to putty. When it has been twisted and then let go, it does not 

 come back to its old place, so that though it is much more 

 largely twisted than wire by the application of a force, it is 

 not possible with accuracy to measure that force. There is, 

 or rather I should say there was, no material that could be u'jed 

 as a torsion thread finer than spun glass ; and therefore physi- 

 cists use instead a fibre almost free from torsion. A single thread 

 of silk as spun by the silkworm is taken and split down the 

 middle, for it is really double, and one half only is used. This 

 is far finer than spun glass, and being softer in texture, it is 

 much more easily twisted. Silk is ten thousand times more easily 

 twisted than spun glass. So easily twisted is silk that in the 

 majority of instruments the stiffness of the silk is either of no con- 

 sequence at all, or at any rate it only produces but the slightest 

 disturbing effect. Now if it is necessary to push the investiga- 

 tion further still by the continued increase in the delicacy of the 

 apparatus, silk itself begins to prevent any progress. Silk has a 

 certain stiffness, but if that were always the same it would not 

 matter ; but then it possesses that putty-like character of spun 

 glass, but in a far higher degree ; it is affected by every variation 

 of temperature and moisture, and any really delicate measures are 

 out of the question when silk is used as the suspending fibre. 



This, I believe, is a fairly accurate account of the state of the 

 case three years ago. At that time I was improving, or 

 attempting to improve, a certain class of apparatus of which I 

 shall have more to say presently, and I was met by the difficulty 

 that a greater degree of delicacy was required than was possible 

 with existing torsion threads. Silk would have entirely prevented 

 me from reaching the degree of delicacy and certainty in this 

 instrument that I hope to show this evening that I have attained. 



Being then in this difficulty, I was by good fortune and 

 necessity led to devise a process which I propose at once to show 

 you. I shall not describe the preliminary experiments, but 

 simply describe the process as it stands. There is a small cross- 

 bow held in a vice, and a little arrow made of straw with a 

 needle point, and I have here a fragment of rock crystal which 

 has been melted and drawn into a rod. It requires a tem- 

 perature greater than that developed in any furnace to melt this 

 material so that it may be drawn out. If the arrow, which also 

 carries a piece of the quartz rod, is placed in the bow, and if 

 both pieces are heated up to the melting-point and joined together, 

 and then the arrow is shot, a fibre of quartz is drawn — that is to 

 say, it is drawn if there is not an accident. 



The arrow has flown, and there is now a fibre, not very fine 

 this time, which I shall hand to our President. At the same time 

 I can pass him a piece of much finer fibre, made this afternoon, 

 which shows (and this is a proof of its fineness) all the brilliant 

 colours of the spider line when the sun shines upon it, but with 

 a degree of magnificence and splendour which has never been 

 seen on any natural object. 



The mam features of these fibres are these. You can make 

 them as fine as you please ; you can make them of very con- 

 siderable length ; you can make pieces 40 or 50 feet long, 

 without the slightest trouble, at almost every shot. Even 

 though of that great length, they are very uniform in diameter 

 from end to end, or, at any rate, the variation is small and per- 

 fectly regular. The strength of the fibre is, I think I may safely 

 say, something astonishing. Fibres such as I have in use at 

 the present time in an instrument behind me are stronger than 

 NO. 1094, VOL. 42] 



ordinary bar steel : they carry from 60 to 80 tons to the square 

 inch. That is one of their most important features, for this 

 reason — that on account of their enormous strength you can 

 make use of very much finer fibres than would be possible if they 

 were not so strong ; and I have already explained the import- 

 ance of the fineness of the fibre when delicacy is of the first 

 importance. 



As to the diameter of these fibres, I have said they can be 

 made as fine as you please. I shall not trouble you with 

 a large number of figures, but one or two may probably be 

 interesting to those who are in the habit of using philosophical 

 apparatus. In the first place, a fibre a great deal finer than a 

 single fibre of silk — that is, one five-thousandth of an inch in 

 diameter — will carry an apparatus more than thirty grains in 

 weight. I have in one of the pieces of apparatus which I 

 shall use presently a fibre the fifteen-thousandth of an inch in 

 diameter. That is, so fine that if you were to take a hundred 

 of them and twist them into a bundle you would produce a com - 

 pound cable of the thickness of a single silkworm's thread. I 

 do not mean the silk used for sewing that is wound on a reel, 

 because that is composed of an enormous number of silk threads ; 

 but a single silkworm's thread as it is wound from the cocoon, 

 and that fibre is at the present time carrying a mirror the move- 

 ments of which will presently be visible in all parts of this large 

 room. 



But that is by no means the limit of the degree of fineness 

 which can be reached. A fibre the fifteen-thousandth of an 

 inch in thickness is quite a strong and conspicuous object. You 

 may go on making them until you cannot see them with the 

 naked eye. You may go on following them with the microscope 

 until you cannot see them with the microscope — that is to say, 

 you cannot find their end — they gradually go out. The ends 

 are so fine that it is impossible ever to see them in any micro- 

 scope that can be constructed, not because the microscopes are 

 bad, but because of the nature of light. But that is a point 

 upon which I shall not say more this evening. It has been 

 estimated that probably the ends of some of these are as fine as 

 the millionth part of an inch — I do not care whether they 

 are or whether they are not, because they can never be seen and 

 never be used — but certainly the hundred-thousandth of an inch 

 is by no means beyond the limit which can be obtained. As 

 these large numbers of hundreds of thousands and millions are 

 figures which it is impossible for anybody thoroughly to realize, 

 I may for the purpose of illustration say that, if we were to 

 take a piece of quartz about as big as a walnut, and if we could 

 draw the whole of that into a thread one hundred-thousandth of 

 an inch in diameter — threads which can certainly be produced — 

 there would be enough to go round the world about six or seven 

 times. 



These quartz fibres, on account of their fineness, are emi- 

 nently capable of measuring minute forces— that is to say, 

 they would be capable if they were free from that putty-like 

 quality which I have described as making spun glass useless. 

 Now, experiments made both in this country and in Australia 

 show that to a most extraordinary degree they are perfectly free 

 from that one fault of spun glass. 



The number of useful properties of quartz that has been 

 melted is so great that I can merely take, in a more or less 

 disjointed way, one or two ; and I propose, in the first place, 

 to say something which, I think, may be especially interest- 

 ing to chemists, and, perhaps, to our President. I should 

 like to ask experimental chemists what they would think of a 

 material which could be drawn into tubes, blown into bulbs, 

 joined together in the same way that glass is joined, drawn out. 

 attached to a Sprengel pump, sealed off with a Sprengel 

 vacuum, which would be transparent, which would be less acted 

 upon than glass by corrosive chemicals, and which, finally, 

 at the point at which platinum is as fluid as water, would 

 still retain its form. Here is such a tube with a bulb 

 blown at the end. I have found that it is possible to make 

 tubes (though it cannot be done in the ordinary way, as with 

 glass) and to blow bulbs with quartz, and that they have this 

 advantage which glass does not possess — namely, that it is 

 almost impossible to crack them by the sudden application of heat. 



Then there is another property which quartz fibres and rods 

 possess which I shall be able to show only imperfectly— namely, 

 the power of insulating anything charged with electricity under 

 conditions under which in general insulation is impossible. 

 You now see upon the screen an electroscope, the leaves of 

 which were charged at noon, and they are still divergent, but 

 not to a very great extent, because they have suffered from un- 



