The Use of Fused Silica

Formerly made only in rod and tube form, fused quartz is now often employed as a substitute for glass in chemical ware, and most of the common pieces used in chemistry are now obtainable in this material. Such articles as flasks, beakers, dishes, plates, and so forth, are in fairly common use.

Apparatus made from fused quartz has two chief advantages over that made from glass. The low thermal expansion coefficient eliminates all fear of breakage due to rapid temperature changes. A hot piece of quartz plunged into water suffers no ill effects. Also, its relatively high melting point makes possible the study of reactions that would be more difficult with glass.

As will be pointed out later, many of its properties make it valuable in instruments of various kinds and when constancy is a prime requisite. One particularly valuable property of fused quartz is its extremely low loss of energy due to internal friction when stresses are applied. The loss amounts to only of that in the best of the metals. Another property of value lies in its constancy of length. It not only has an extremely small thermal expansion coefficient, but returns to its original length after having been heated or cooled.

The chief disadvantage of fused quartz is its high cost, due mainly to the fact that it has a high melting point and demands special methods for its manufacture. The fact that it fuses with difficulty makes the working of tubing greater than an inch in diameter more or less impractical in the laboratory. Although an oxyhydrogen flame becomes useful when working large pieces of quartz, small pieces can be worked easily with an oxy-natural gas flame. An ordinary Bunsen burner flame using natural gas is hot enough to soften small pieces.

A very useful property discovered by C.V. Boys in 1889, and dicussed in detail later, is that fused quartz can be drawn into fine fibers which have remarkable strength. Fibers of any size down to 1 micron (0.0001 cm) diameter or less can be easily and rapidly produced. No other vitreous material can in any way approach fused quartz in performance when made into these fine fibers.

Chemica1 properties

Fused silica at room temperature is inactive to practically all chemicals except hydrofluoric acid and the alkalies. However, at high temperatures it reacts with most metallic salts, forming silicates. This is due to the fact that silicon dioxide is an acid in the general sense of the term, and as such reacts vigorously at high temperatures with metallic oxides which are bases. The noble metals do not form silicates, and a quartz fiber covered with gold may be heated until the gold evaporates, without harming the fiber.

Thermal properties. The coefficient of thermal expansion of fused quartz rod under no stress has been measured with considerable accuracy. The mean values near room temperature, defined by are given in Table I. For comparison, steel has a coefficient of , or 25 times as large, while for Invar is about . The coefficient of thermal expansion has not been measured for various sizes of fibers under varying amounts of strain.

TABLE I

The coefficient of thermal hysteresis of fused quartz is less than for any other known material. If a substance of length I is heated from a temperature to a temperature and allowed to cool to t then , where is the residual difference in length, is a measure of the thermal hysteresis. For quartz, this quantity is -1 to ; that is, it contracts more than it expands. In comparison, Invar has a similar coefficient of . This property makes fused quartz particularly valuable when it is necessary to maintain dimensions accurately.

If fused quartz is held at a temperature above 1200 degrees C for some time, crystallization gradually takes place, beginning at the surface and working inward. As the temperature is raised, the crystallization becomes more rapid until a temperature is reached at which the crystals melt. When quartz is worked locally in a flame, a milky surface will form between the soft quartz and the cool portion. This is probably due to condensation of evaporated quartz and does no harm to the material except in appearance.

Elastic properties. The normal coefficient of elasticity, or the reciprocal of Young's modulus for quartz rod at room temperature, was measured first by Boys. This coefficient is defined by

where Y is Young's modulus and is the normal stress. Emperically, , which is very near the most recently determined values for fibers from 50 to lOO microns in diameter. Young's modulus varies with the size of the fiber, becoming greater as the size of the fiber diminishes. This variation can be expressed by

where d is the diameter of the fiber in microns. This relation fails to hold, giving values too large, for fibers less than l0 microns in diameter. Experimental values of Y for various sizes of fibers are given in Table II. The increase in modulus of elasticity with decrease in size is due to the importance of the surface layer for the smaller fibers, which has a different elastic constant.

TABLE II

The tangential coefficient of elasticity, or the reciprocal of the rigidity modulus, for solid rod of radius r and length 1, is defined as

where , is the tangential stress and is the angle of twist of the rod. For a uniform solid round rod , where L is the applied torque and r is the radius. Z has a minimum value of but depends, as does Y, on the size of the fiber, as shown in Table II.

Two other elastic quantities are very often useful. The first indicates how much a fiber can be stretched before it breaks, that is,

where is the normal stress for failure. Values of for failure are given in Table II. These apply only to fresh, clean fibers or those which have been kept perfectly clean and dry. (See below as to how to preserve fibers.) As far as is known, no other material approaches this factor. For the best nickel-vanadium steels the ratio is about 0.01. A comparison of Young's modulus for each material shows that quartz fiber compares favorably in strength with the strongest materials known.

The second quantity indicates how much a fiber can be twisted without failure, that is,

for fibers up to 20 microns in diameter, where is the tangential stress for failure. This ratio also increases as the size of the fiber decreases. Thus, a fiber 5 microns in diameter can be twisted through at least 20 revolutions per centimeter of length before it fails. It should be remarked that the elastic limits for both normal and tangential stresses are coincident with the point of failure.

Another property of quartz which enhances its value for electrometer and other suspensions is its low internal viscosity. If a fiber is twisted through an angle , then the shearing stress is not strictly a constant but depends on time, thus:

The coefficient is a measure of the internal friction, or viscosity. Some representative values are given in Table III.

TABLE III

If a fiber of length l and radius r is allowed to oscillate in a vacuum with a body of moment of inertia I suspended from the lower end, and if T is the period and the logarithmic decrement of the vibration, the coefficient of viscosity in poises is given by

If such a torsion pendulum has a period of 2 seconds, it will lose about 10 per cent of its amplitude in 24 hours.

Thus as defined above, should be as small as possible if the internal losses are to be kept at a minimum.

Thermal-elastic properties. Both Young's modulus and the rigidity modulus for fused quartz depend on temperature. Each becomes greater with moderate increase in temperature. Boys gives the coefficient of Y is , and for Z it is the same. For very accurate work any instrument using quartz fiber should be calibrated at more than one temperature.

Hardness. Fused silica has a hardness of 7 on the 1 to 10 scale. It is thus harder than glass and also harder than most of the metals.

Surface tension of molten silica. If a fiber is heated until the quartz becomes quite soft, it will tend either to shrink and enlarge at the point of heating or to pull apart, depending on the tension. We may define the surface tension as the force per unit of circumference tending to pull the fiber together. This varies with the temperature, but an average value will be 250 dynes . In comparison, glass has a surface tension of 140 to 160 dynes .

Electrical properties. When fused quartz is clean and dry, it is probably the best electrical insulator known. For this reason it is useful in such apparatus as electroscopes and electrometers, in which leakage must be reduced to a minimum. If used in the open air, quartz covered with the wax known as ceresin is still better than amber as an insulator. Care should be taken that the ceresin is that distilled from the natural mineral and not the synthetic material very often sold. When it is applied, the temperature of both the quartz and the ceresin should be from 80 to 100 degrees C. for the first dip. Thicker coatings can be applied by allowing the quartz to cool before dipping again.

The absorption of electrical charge, or "soak-in," is extremely low, being less than 10 per cent of that for amber.

The remarkable property of retaining and even increasing its strength as it is drawn into fine fibers makes the number of applications of

quartz to fine instruments many and varied. Few scientists, it seems, have realized and appreciated its values. Stronger than any of the metals used for suspensions, with the exception of tungsten, it has the advantage that it can be made according to the specific requirements. Although some practice is necessary to acquire the proper skill, its acquisition would seem eminently worth while, considering the results that can be obtained.

A description of the torch burning natural gas and oxygen used by the author of this chapter will be given. If other gases are used, it may be necessary to modify the technique given below to meet the specific conditions.

Fig. 1. Large and small torches for working fused quartz.

The torch is made from a piece of brass tubing bent into the shape shown in Fig. 1 and having one end threaded for removable tips. The best size of opening for quartz work is about 2 mm in diameter. Other sizes of tips from 1 to 3 mm in diameter will be found useful. To produce the necessary long steady flame, the length of the hole in the tip should be at least five times its diameter. The oxygen and gas are mixed at some distance from the torch. An ordinary T is sufficient for this mixing. It is necessary to have a ready means of control for both the gas and the oxygen. If the latter is under high pressure, a reduction valve in conjunction with a needle valve gives the best regulation. A combination of needle valves and T which has been found to give satisfactory service is shown in Fig. 2.

Fig. 2. Combination of needle valves and mixer

In using such a torch, care should be taken in lighting to turn the gas on first, light it, and then gradually turn the oxygen on until the proper flame is produced. To extinguish the flame, turn the oxygen off slowly and then the gas. Disregard of this procedure may result in a backfire into the line but usually does little damage except to sensitive nerves.

The described torch is a useful adjunct to any laboratory, especially when supplied with tips of various sizes. It is ideal for working Pyrex glass as well as quartz. When quartz fibers are being made, the torch is held by a clamp so that the flame is vertical.

Indispensable in the working of small pieces of quartz is a small torch shown in Fig. 1, identical with the larger one except for size, and using the same gases, which are controlled by separate fine needle valves. The best metal tubing for this torch is brass or copper 1/16 inch in internal diameter. The gases are led from the mixer to the torch by 1/16-inch rubber tubing. Small volumes throughout are important, or much time will be wasted in waiting for a change of gas mixture to arrive at the tip. The tips should be interchangeable and should have openings of from 0.05 to 0.2 mm in diameter. A slight modification of design (illustrated) permits the torch to be mounted and manipulated by mechanical means. The usefulness of this small torch will become apparent later.

In measuring the sizes of fibers, an ordinary microscope equipped with a scale in the eyepiece and having a magnification of from 300 to 1000 is very useful. With some experience the sizes of fibers can be judged to within 20 to 50 per cent by the amount of scattered light, the way they weave in the air, and so forth, but in many cases the diameter is important, and an accurate means of determining their size is invaluable.

After blowing out a fine fiber, two places are marked, and the position of the intervening portion is thus determined by small tabs. Dennison's No. 251 tabs are recommended.

Fig. 3. A simple fork used for holding fibers while they are being mounted

In many instances one works with fibers from a few centimeters to 10 or even 20 cm in length. In these cases 3. the fibers are mounted on a two-pronged fork. This is | easily made as shown in the sketch, Fig. 3. The end of each prong is drilled, and a piece of quartz (50 microns to 150 microns) is put in with hard wax. The reason for the quartz tip is to allow some freedom to the fiber, since the quartz tips will bend if the fiber is pulled one way or the other. Rigid supports result in many more broken fibers. The fiber is fastened to the tips with a small piece of hard wax.

In cases in which one fiber is melted to another, each will shrink, and the quartz will gather at the junction. It is necessary then to have two forks, each with movable prongs. The fork designed according to Fig. 4 has proved very satisfactory. If the handles are attached at different angles, the two forks can be worked together more easily.

Fig. 4. Adjustable prong fork

A hot wire mounted as in Fig. 5 has many uses and is especially valuable in melting small pieces of wax. The resistance wire can be any one of several, such as platinum, German silver, Chromel, Nichrome, and so forth. It should be 24-26 B and S gauge. A toy transformer with variable voltage of from 1 to 6 volts is convenient for controlling the temperature. A foot switch is very useful, since both hands may be occupied when the heat is wanted.

In testing for conductivity of quartz fibers which have coating of metal, a probe (see Fig. 6) with a fine platinum wire tip finds a use. For such testing high voltages should not be used, since the resulting sparking will remove the metal from the fiber around the point of contact. Several volts applied through a 100,000-ohm resistance and a low-sensitivity galvanometer will be found satisfactory for qualitative work.

Waxes are indispensable in fastening fibers either temporarily or permanently. For general use Dennison's hard red wax, DeKhotinsky wax, or flake shellac is recommended. If the wax is holding in place two or more fibers which are to have a metal evaporated or sputtered onto them, one of the latter two waxes should be used and heated until polymerization takes place, resulting in a material either difficult or impossible to melt. Otherwise the heat developed during the process of depositing the metal may cause the wax to soften and the fibers to be displaced.

Fig. 5. Hot-wire holder

In case it is necessary to hold a fiber temporarily and to maintain its desirable qualities, a wax must be used which, when heated, will completely disappear and not react in any way with the quartz. None of the products sold as waxes serve the purpose. An organic chemical which has the desired properties is diphenylcarbazide. It usually comes in powdered form and should be as pure as possible and especially free from inorganic materials.

In handling small pieces of wax, holding fibers, bending quartz fibers; and so forth, a piece of quartz lOO microns in diameter and 2 to 3 cm long, waxed into the end of a metal rod, is very useful (See Fig. 7.) It will also be found that a needle mounted in the end of a metal rod has many uses. It is recommended that several such quartz and needle holders be available.

Fig. 6. Platinum probe for testing conductivity of metal-covered fibers

When working with small objects, tweezers of various sizes are very convenient. These can be obtained from jeweler's supply houses or from most houses supplying scientific apparatus. For very fine work, watch-hairspring tweezers such as #3C made by Dumont & Fils, Switzerland, are recommended. Also valuable in cutting fibers are small scissors. These may be a good grade of manicure scissors or dissecting scissors used in biological work. A nick should be made in one blade to prevent large fibers from slipping. If the

scissors are guided by mechanical means, small fibers (up to 40 mictons) can be cut off as little as 0.01 mm at a time under a microscope.

Fig. 7. Various instruments useful in fiber work

In most fiber work it is necessary to fix the position of the fiber with some accuracy. Small tripods with adjustable feet, together with clamps and rods, as shown in Fig. 8, will serve to hold the various forks, needles, and so forth, used in the process of mounting the fibers. It is very difficult to hold a fiber still enough by hand, and it is always best to take advantage of mechanical devices wherever possible.

Very small fibers (1 micron and less) can be easily seen by scattered light against a black background. Black velvet is one of the best. If the diameter of a fiber is to be measured under the microscope, a light background is needed; the scattered light against black gives a false impression of the size, since the actual outlines of the object cannot be seen.

Fig. 8. Support for holding work or fixing the position of fibers

To put a conducting coat of metal on quartz, any one, of several methods can be used. The simplest, and one which is satisfactory for fibers down to 20 microns in diameter, is to bake the metal on, using any of the good china paints. Most of the noble metals—for example, platinum, gold, iridium, and so forth—can be obtained in this form. The paint is-made by dissolving one of the metal salts in an organic liquid. China painters use this on their dishes and fire them to 700 degrees C. The organic material disappears, and the metal compound decomposes, leaving behind a uniform coating of the metal. The thickness for each coat may vary from 0.05 microns to 0.15 microns , depending on the thickness of the original paint. Very adherent, electrically conducting coatings can be applied to glazed p8arcelain, glass, quartz, and so forth. The hot wire, held under small pieces of quartz fibers covered with these solutions, will bake them in a few seconds. If an attempt is made to treat small fibers in this way, it will be found that the solution collects into small drops along the fiber, and a disconnected coating results when it is baked.

Sputtering or evaporating the metal on are the most satisfactory methods and have the advantage that conducting coats can be applied to fibers of any size. In general it is desirable to arrange to coat the fibers on all sides. Evaporation is the easier and simpler of the two methods. A suitable apparatus for this is shown in Fig. 9.

Fig. 9. This arrangement allows the evaporated metal to be deposited on all sides of the work

In working quartz it is absolutely necessary to use dark glasses to protect the eyes. Besides the brilliant glow, which in itself is bad for the eyes, the light is very rich in ultraviolet, which is especially harmful and may cause blindness through long exposure. The glasses should be gray in color, preferably, and have a transmission of from 10 to 20 per cent. Ordinary glass will cut out the ultraviolet, so that inexpensive dark glasses will suffice.

The writer has used for some time a set of three micromanipulators. Each has a three-jointed arm, which allows complete freedom in determining the position of the fiber. For fine adjustment, micrometer screws with divided heads give accurate motion in three mutually perpendicular directions. The accompanying illustration, Fig. 10, shows one of the three manipulators.

Fig. 10. Micromanipulator

Although much of the simpler fiber work can be done with the unaided eye or with a magnifying glass, for fine work in which accuracy is important and ease of working is desired a binocular microscope with a magnification of 15 to 20 can be strongly recommended. Such a microscope not only gives stereoscopic vision but when used properly results in little, if any, eyestrain. A scale in one eyepiece allows measurements to be made. Lighting from several directions is desirable to provide proper illumination on the work in all positions.

A complete setup of the major equipment used by the writer in quartz fiber work is illustrated in Fig. 11 The black glass base permits the fine fiber to be seen easily by scattered light. When the actual outlines of large fibers are to be seen, a piece of white paper is placed on the glass base and used as a background.

Making fibers. A convenient size of stock quartz rod is 3 to 4 mm in diameter. Smaller rod than this is apt to break when the larger fibers are being drawn and is not easily held in the hands. Larger rod becomes more difficult to melt.

Fig. 11. Complete assembly for working quartz fibers

The first step in making a fine fiber is to draw one from 50 to 100 microns in diameter. (See Fig. 12.) Two pieces of stock quartz of convenient length are held in the hands. The oxygen-gas flame is adjusted to maximum heat; that is, both the oxygen and gas are increased, especially the oxygen, until a hissing flame results, and the small cone just over the opening in the torch tip has shortened until its height is perhaps two or three times its width. The hottest portion of the flame is just above this small cone. The ends of the quartz rod are melted together and then pulled apart a short distance, so that the connecting soft quartz is perhaps 1 mm in diameter. This portion, when held in the hottest part of the flame, will become quite soft. The quartz rods are then quickly removed from the flame, and at the same time the two pieces held in the hands are separated rapidly to a distance of several feet. The hotter the narrow section of quartz and the faster the drawing, the smaller will be the resulting fiber. Fibers down to 20 microns can be drawn in this manner.

Fig. 12. The first step in making a small fiber is to draw a larger one. A very hot flame is used

To make a smaller fiber from the larger one, the procedure is as follows: Break the connecting fiber produced in the above drawing process so that a section of 8 to 10 inches is left on each piece of quartz stock. This section should be stiff enough to support itself in a vertical position. Now adjust the flame by turning the oxygen partially off, so that a steady flame about 15 to 20 inches long is produced. The cone above the tip will lengthen to several inches Holding the quartz stock so that the attached fiber is vertical, move it into the vertical flame as illustrated in Fig. 13. The whole length of the fiber will glow uniformly. If the temperature of the flame and the size of the fiber are right, the fiber will gradually begin to lengthen, slowly at first and then more rapidly as it becomes smaller. Finally, the upper section of the original fiber will go quickly toward the ceiling. As soon as this happens, the lower end should be removed ~ from the flame. A careful examination will reveal a fine 31 fiber joining the two ends of the original, perhaps 3 to 6 feet long. Sections of it can be seen in scattered light. Place a small tab on one part of the fine fiber with one hand while holding the stock quartz (to which the other end of the fiber is attached) in the other. The position of the intervening portion is now determined, so that other tabs can be stuck on and suitable lengths removed. Each end of each length will thus have a small tab attached. These fibers are then stored in a clean container in which the air is kept dry. (See figure 14.)

Fig. 13. The second step in making a small fiber is to blow out the large fiber by holding it in a long, vertical, relatively cool flame

The size of the resulting small fiber will depend on a number of factors. Chief among these are the size of the original fiber, the temperature and size of the flame, and the time intervening between the disappearance of the top of the original fiber and the removal of the lower end. Some practice is necessary to secure fibers of a desired size. It will be found that fibers produced in the above manner are straight and of quite uniform diameter for some distance on each side of the center.

A few cautions are necessary if good fibers are to be had. The basis of all of these is cleanliness. Much of the dust on objects around a laboratory and floating in the air is inorganic. If a fiber is heated where a piece of dust has settled, the metallic salts form silicates and in general completely spoil the surface, and for that reason the fiber also, at the point of contact. It is a general rule that no part of a fiber which ultimately is to have any stress applied should ever touch anything except those materials which are softer than the quartz and will not react with it. This may seem to be a stringent requirement, but in reality the fiber can always be handled by its ends, which are eventually discarded.

If the original large fiber shows any bright spots when put into the flame, it should be discarded. In general, this is the best test for dust that can be applied. Dust will immediately show itself by causing a bright spot, and the fiber can be discarded forthwith; if there is no dust on the fiber, it will not be harmed by heating. This test can be made with fibers from 10 to l00 microns with an ordinary Bunsen burner. For smaller ones the small torch using a pure gas flame should be used. In each case the fiber should be under some tension to keep it straight.

If the size of the fiber is to be measured with the microscope, it is usually sufficient to take a sample from each end and take the mean diameter. The sample is placed on a piece of glass, which in turn is placed on the microscope stage and viewed by transmitted light. To find the fiber in' the microscope the following procedure is valuable in saving time: Have plenty of light passing through the optical system. Raise the objective until it is several times the working distance from the object. Remove the ocular. Move the glass on which the fiber is lying until, by looking down the microscope tube, the reduced image of the fiber is seen. Adjust the position of the fiber until its image appears approximately in the middle of the objective. Now move' the objective down until the image begins to spread. When it appears to cover the objective completely, the object is near the focus, and on replacing the ocular, the image should be in the field of view.

After working with fibers for a while, one can judge their size by the amount of scattered light, the amount of weaving in the air, how much a fiber of a given length sags under its own weight, the radius of curvature when hung over a needle with a tab on one end, and so forth. These methods are good to from 20 to 50 per cent, except for fibers below 1 to 2 microns.

Another method for drawing fibers has been described by Boys. It consists in pulling the two pieces of quartz apart very rapidly by means of a projected arrow. Long fibers down to 10 microns of very uniform diameter can be produced in this fashion. The hotter the quartz and the faster the arrow is shot the finer will be the fiber.

Fig. 14. Preserving quartz fibers

The care and preservation of small fibers. When a fiber has its two ends marked with tabs, it should be hung in a clean, dry container. A crosspiece at the top of the container, on which are small pieces of soft wax or beeswax, serves as a hanger. The top tab is pressed into the wax, and the lower tab keeps the fiber from weaving around and touching things.

The container should be 10 to 12 inches deep, airtight, and preferably made from glass. It should be clean and contain a good drying agent either phosphorous pentoxide or anhydrous potassium hydroxide. A convenient container is made from an inverted bell jar with a plate-. glass top as shown in Fig. 14. Fibers deteriorate in moist atmospheres, but can be preserved for months with no change in breaking strength if kept clean and dry.

Some useful techniques in fiber work. Straightening. Fibers from 10 to 500 microns can be quickly and easily straightened by hanging a weight on the lower end and running a Bunsen burner flame up and down the piece several times The weight should be somewhat less than that necessary to elongate the fiber appreciably under the heat of the flame. A small Dennison tab is sufficient for fibers 10 to 50 micron and a 2-inch tab for those between 50 and 500 microns. For fibers from 4 to 10 microns a small Dennison tab should be cut in two and the small torch burning pure gas used for heat.

Bending. Fibers from 40 microns on up are best bent by hanging a weight such as a tab at one end, holding the fiber at the proper angle, and applying the heat locally with a small torch burning oxygen and gas. The piece between the flame and the tab will fall to a vertical position as shown in Fig. 15.

Fig. 15. Bending large and small fibers

Fibers between 1 and 40 microns are best bent over another piece of quartz. A weight such as part or all of a small tab or a small piece of wax bends the fiber over the larger piece of quartz (100 microns or less). A pure gas flame applied with the small torch at the contact of the two fibers will bend the smaller one over the larger. The flame should not be applied longer than is necessary, or the two pieces of quartz are apt to stick together.

Drawing and shrinking. If one end of a fiber is attached to a screw-controlled sliding mechanism, such as the movable prong fork described earlier, a portion of it may be readily drawn down to any desired size by applying a flame with the small torch and gradually screwing out one prong.

Fig. 16. Shrinking a small fiber

Soft quartz has a high surface tension, and fibers tend to shrink when heated. The heating is done with the small torch. It is necessary to have a properly adjusted flame. A compromise must be made between a hot flame with swiftly rushing gases, which readily melts and blows the fibers apart, and a cooler flame, which will not soften the quartz sufficiently. The ideal is reached when the tendency to blow away is overcome by the tendency to pull together due to surface tension. The fiber is heated in a slackened condition, and as the shrinkage proceeds it is fed by the movable prongs. A torch tip with a hole about 0.1 mm in diameter is perhaps the best. With some practice a fiber may be locally enlarged to many times its previous diameter. (See Fig. 16.)

Joining one fiber to another. When the above technique has been learned, the joining of two fibers crossing one another becomes simple. Each shrinks to the common junction, forming a joint which is stronger than any other portion. For this work it is necessary to use two of the forks with movable prongs, gradually feeding in the quartz as the joint grows 1n size.

Joining a fiber to a larger piece of quartz. If the larger piece is too large to melt locally with the small torch, a "teat" is put on at the proper place with a larger torch and then drawn down to a fine point. The fiber, mounted on the fork, is placed next to this teat, and heat is applied to the teat. Upon softening, the larger piece of quartz draws the small fiber in by surface tension. Straightening of the fiber near the junction is done by heating with the small torch burning pure gas when the fiber is under a slight tension.

With care, fibers as small as 1micron in diameter can be melted to other fibers or larger pieces of quartz.

Fig. 17. Making a flat fiber.

Drawing an oval fiber. The tip of each piece of the stock quartz is heated in the oxygen-gas flame so that only the very end becomes soft. With the axes of the two pieces held parallel, the ends are brought together and immediately separated at right angles to the axes of the stock quartz, and at the same time they are removed from the flame. (See Fig. 17.) Only flat fibers larger than 30 to 40 microns can be produced in this fashion. They are useful in vibration types of pressure gauges in which the motion is to be limited to one plane.

Drawing fat tubing. In some cases quartz is useful in making the Bourdon type of pressure gauge. If a long piece of flat tubing is made into a spiral and a mirror and scale are used to measure the change in angle, such a gauge becomes an accurate means of measuring moderate pressures. One way to produce long pieces of elliptically shaped thin-walled tubing is to use two large torches as cross-fires and to heat 1/ 2- to 3/4-inch quartz tubing without rotation. Heating should continue until the walls nearest the flame are quite soft. The tubing is removed from the flame and rapidly pulled to 3 or 4 feet. If heating has not been sufficient, the elongated occluded bubbles will cause the resultant tubing to be brittle. It is, in fact, a good procedure to work the heated section by alternately enlarging and contracting it with internal pressure before drawing. The oval tubing is bent into the desired shape with a moderately hot flame.

Fig. 18. Design of quartz fiber support used in the Dolezalek and Compton electrometers. The whole is made from fused quartz, upon which is deposited a coating of metal, for example, gold or platinum.

Making electrometer suspensions. Quartz fibers make ideal e suspensions for electrometers. The most satisfactory way of making the suspensions consists in joining the ends of the fiber to two larger pieces of quartz by melting them together with a small torch. In many cases these larger pieces are bent into small hooks, and then the whole is made conducting by evaporating or sputtering gold or some other metal on it as represented in Fig. 18. In cases in which hooks cannot be used, the larger quartz is left straight and is cemented into place with a hard wax such as DeKhotinsky's. Contact is made by attaching a fine wire to the quartz with hard wax before the fiber is coated with the metal. The wire is later soldered to the metal pieces of the electrometer.

The method of soldering the metal-coated fibers does not produce a suspension as permanent as with the methods described above. The gold is apt to amalgamate with the solder and result in a poor contact between the main portion of the fiber and the solder.

Another method of fastening fibers to metal parts and at the same time making an electrical contact is to use colloidal graphite. A small drop is placed at the proper point, and in a short while the water will evaporate, leaving a strong conducting joint.

Quartz is very convenient for making various types of electroscopes. It is not only good for the moving parts but is used uncoated for insulation.

Mounting cross hairs in optical instruments. Fibers made from quartz surpass any other material for cross hairs. Owing to the refraction of the light by the fiber, it appears black as seen in a bright field. Its essential smoothness, freedom from dust, uniformity of size, straightness, and the fact that it can be drawn to any desired diameter make it especially valuable.

Fig. 19. Steps in mounting cross hairs for microscope and telescope eyepieces

The mounting is first prepared by melting hard wax onto it at the desired points. The fiber is mounted on a fork and lowered into position. A hot wire brought near the wax where the stretched fiber rests will allow the fiber to sink in and become firmly attached. The various steps are illustrated in Fig. 19.

Torsion balance. For objects weighing less than 1 mg the torsion balance becomes very useful. It is not difficult to make a balance having a sensitivity of to g/div. without the use of mirrors or microscopes. A simple calculation will show the size of fiber necessary for the specific requirements. The crossarm should be statically balanced. The amount of twist of the fiber is conveniently read from a divided head.

The balance may be calibrated by weighing on an analytical balance a long section of fine wire such as 40 B and S gauge copper, 2-mil nickel, or smaller if needed, and cutting from this piece samples of a given length. Usually ten samples will give a probable error of less than 1 per cent in the calibration. If the tension in the torsion fiber is kept constant with a quartz bow, it can be assumed with much accuracy that the twist is proportional to the weight. Since is the surface strain, where r is the radius of the torsion fiber, the angle of twist in radians, and l the length twisted, and since the maximum value of this is about 0.05, the maximum load which the balance can handle is easily computed. A simple design of such a torsion balance is shown in Fig. 20.

Fig. 20. Simple design of a quatrz microbalance

If all the joints are made of fused quartz there need be no fear of a changing "zero," since the limit of elasticity coincides with the breaking point.

Other uses of quartz. Quartz rod or fiber is often used as a carrier of light—visible, ultraviolet, or infrared. Internal reflections keep the light inside the quartz and permit it to be led around corners, provided the corners are not too sharp.

In many cases in which accuracy in maintaining shape or position is important, quartz finds a use. All metals change their dimensions with time, especially when under strain. This change can be lessened by thorough annealing, which consists in subjecting the metal alternately to temperatures above and below room temperature. In extreme cases this treatment may take days or weeks. Annealed fused quartz does not suffer from changes in dimensions, since the flow under strain is less than 10^-3 of that for metals.

Fused quartz is finding increasing uses in lamps of various kinds in which the transmission of ultraviolet light is important. For the same reason many photoelectric cells are made from quartz.

Although the above does not pretend to be an exhaustive list of the uses to which fused silica can be put, it is hoped that the reader will gain some idea of the usefulness of this material.

Fused quartz is obtainable from the Thermal Syndicate and the General Electric Company. Each carries a large stock of quartz products and will make special pieces on demand.

Boy's, C.V., Roy. Soc. Phil. Trans., 143, 159 (1889).

Kaye, G.W.C., Phil. Mag., 20, 718 (1910).

For a discussion of the behavior of metals and quartz used as standards of length the reader to Glazebrook, Sir Richard Tetley, editor, Dictionary of Applied Physics, Volume III, pages 471-475. New York: The Macillan Company, 1922-1923.

Honda, K., Phil. Mag., 42, 115 (1921).

Glazebrook, Sir Richard Tetley, editor, Dictionary of Applied Physics, Volume III, page 699. New York: The Macillan Company, 1922-1923.

Ibid., Volume III, page 696.

See "Electrometers and Electroscopes."