Coefficient of simple rigidity.-The torsional coefficient was determined by the well-known method of vibration. The diameter and length of the fibers were carefully measured, a cylinder made of Tobin bronze was suspended by the fibers, and the period of the torsional vibrations determined by means of a chronometer. Then Since it became apparent that different fibers of the same substance gave slightly different values, and since I measured the diameter of the fibers only with an accurate micrometer, I did not take the trouble to correct for the coefficient of expansion." The mean of a number of experiments gave for quartz n=3.4× 10"; for the steatite fiber a value from 2.6 to 4.2x10". Also in this respect the new fiber almost exactly equals fused quartz. The temperature coefficient of the torsional constant of quartz is given by Threlfall as +0.000133; by Barnett as +0.000115; I obtained +0.000149. Steatite does not show the remarkable property of becoming more rigid at higher temperatures. Its temperature coefficient is negative, a= -0.000193. The results obtained are given in Table II and plotted in fig. 1. Coefficient of expansion of fused steatite. The following experiments were made by Mr. L. G. Hoxton, of the Bureau of Standards, using a rather thick fiber of fused steatite. The length between two fairly well defined marks 110.4 mm apart was compared at different temperatures with that of the standard nickel-steel decimeter No. 43, belonging to the Bureau and furnished with a certificate from the Bureau International des Poids et Mesures, which gives its linear coefficient as a=0.96×10°. In the following table the differences between the length of the fiber and the standard are marked F-NS. In the third column is given the linear coefficient of expansion, counting from the lowest temperature, and in the last the number of observations made for each temperature. The average relative coefficient is found from the normal equation 1059.4 410.1X110400 The absolute linear coefficient of expansion is therefore a = 0.0000045. The comparisons were made on a Zeiss horizontal comparator, which allows a direct reading of 1.0μ. An attempt to obtain a fiber with a zero temperature coefficient by melting quartz and steatite together was unsuccessful. As soon as too much quartz is present the bead loses its transparency. If beads made of soapstone should not be clear, it is generally due to an excess of quartz in the substance. In such a case the addition of a small amount of magnesia or magnesium carbonate will clear up the bead. Inder of refraction of fused steatite.-Two small prisms were ground from steatite beads, and their index of refraction for sodium light determined by the method of minimum deviation. The instrument used was a spectrometer reading to half minutes. contains the results. Table IV Also for these determinations I am indebted to Mr. L. G. Hoxton. From the above comparison we conclude that we have found in fused steatite a substance which shows all the characteristic properties. of fuzed quartz. The method of drawing fibers from it is very simple, and since the substance is easily secured (an old jet of a gas burner will furnish a large number of fibers), these fibers may, with advantage, be used where economy of time is an important question. One more advantage lies in the easy handling of the fiber. While thick quartz threads break easily when bent, those of steatite may be bent considerably more without breaking. Barnett working with fibers from 0.005 to 0.007 cm in diameter did not detect any time effect upon the period. But I found a distinct effect with either quartz or steatite fiber. The quartz fibers became a Barnett: loc. cit., p. 116. more rigid in course of time, a behavior resembling that of metals." Professor Carhart and the writer' have found, however, a decrease of rigidity in the case of phosphor bronze. Continued vibrations, especially of very large amplitude, seem to have a distinct influence upon this "settling down" to a steady state, if such can be obtained. Tests have been made concerning this time change in order to determine whether different specimens of quartz will differ in their behavior-mine contained quite a large amount of strontium-or whether the difficulties are to be attributed simply to the size of the fibers. Fibers were drawn from a stick of quartz glass (Heraeus). These show no time effect if the load is very small in comparison with the tensile strength, but the time effect becomes more and more pronounced the larger the load. After a fiber 0.02 cm in diameter had been vibrated for about a week with a load of 400 grams, the latter was removed and the fiber allowed to rest for a couple of days. It showed a partial return to the original state. It seems that the thickness of quartz fibers can not be increased beyond 0.002 or 0.005 cm without the loss of the desirable properties which make them so well adapted for very fine suspensions. a Nichols and Franklin: Elements of Physics, I, p. 99. |