180 
MR. A. A. GRIFFITH ON 
6. The Strength of Thin Fibres. 
Consideration of the consequences of the foregoing general deduction indicated that 
very small solids of given form, e.g., wires or fibres, might be expected to be stronger 
than large ones, as there must in such cases be some additional restriction on the size 
of the flaws. In the limit, in fact, a fibre consisting of a single line of molecules must 
possess the theoretical molecular tensile strength. In this connection it is, of course, 
well known that fine wires are stronger than thick ones, but the present view suggests 
that in sufficiently fine wires the effect should be enormously greater than is observed 
in ordinary cases. 
This conclusion has been verified experimentally for the glass used in the previous 
tests, strengths of the same order as the theoretical tenacity having been observed. 
Incidentally, information of interest has been obtained, somewhat unexpectedly, 
concerning the genesis of the flaws, and it has been found to be possible to prepare 
quite thick fibres in an unstable condition in which they have the theoretical strength. 
Fibres of glass, about 2 inches long and of various diameters, were prepared. One 
end of a fibre was attached to a stout wire hanging on one arm of a balance, and the 
other end to a fixed point, the medium of attachment being sealing wax. A slight 
tension was applied while the wax was still soft, in order to eliminate bending of the 
fibre afc the points of attachment. The other arm of the balance carried a beaker into 
which water was introduced from a pipette or burette. The weight of water necessary 
to break the fibre was observed, and the diameter of the latter at the fracture was 
found by means of a high-power measuring microscope. Hence the tensile strength 
was obtained. 
At first the results were extremely irregular, though the general tendency of the 
strength to increase with diminishing diameter was clear. It was found that the 
irregularities were due to the dependence of the strength on the following factors — 
(1) The maximum temperature of the glass.—To secure the best results it was found 
necessary to heat the glass bead to about 1400° C. to 1500° C. before drawing the fibre. 
(2) The temperature during drawing.—-If the glass became too cool before drawing 
was complete, a weak fibre was obtained. This temperature could not be very closely 
defined, but it is perhaps the same as the limiting temperature of the viscous liquid 
phase, namely, 730° C. This effect made the drawing of very fine fibres a matter of 
some difficulty, as the cooling was so rapid. 
(3) The presence of impurities and foreign bodies. 
(4) The age of the fibre.—For a few seconds after preparation, the strength of a 
properly treated fibre, whatever its diameter, was found to be extremely high. Tensile 
strengths ranging from 220,000 to 900,000 lbs. per sq. inch were observed in fibres up 
to about 0-02 inch diameter. These strengths were estimated by measuring the radii 
to which it was necessary to bend the fibres in order to break them. They are therefore 
probably somewhat higher than the actual tenacities. The glass appeared to be 
