referring now to the drawings , fig1 depicts schematically the use of the present invention in its most preferred embodiment . a suitably supported measuring apparatus ( 1 ) is used to measure a length of strand ( 2 ) unwound from a rotating forming package ( 3 ) onto a bobbin ( 4 ). as the strand is unwound , a twist is imparted to it by means of a rotating traveler ring ( 5 ) as it would be done on a conventional textile twist - frame . as the strand ( 2 ) is unwound from the supply source and onto the bobbin ( 5 ), it is brought into contact with the outer surface of a rotatable metering rim ( 6 ) housed inside the measuring apparatus ( 1 ). the strand is passed through an opening or guideway ( 7 ) and along the inside surface of a conically shaped stationary strand threading guide ( 8 ). the strand is then partially wrapped about the circumference of the metering rim ( 6 ) through a contact angle of up to about 315 degrees . this partial wrap insures continuous contact of the strand against the surface of the metering rim . at least two adjustable positioning guides ( 9 and 10 ) are also arranged such that the angle at which the strand first contacts the rim ( 6 ) and later leaves its surface can be controlled by the positioning of the guides . these adjustable strand positioning guides are further illustrated in fig2 - 4 . in the preferred embodiment , they are generally made of free machining brass although any material resistant to the cutting action of glass and which does not itself adversely affect the integrity of the strand may be used . the upper guide ( 9 ) is preferably bent backward in the shape of an elongated hook or &# 34 ; j &# 34 ; so that the strand ( 2 ) can be easily inserted . the elongated opening of the upper guide ( 9 ) allows the strand to oscillate back and forth in a plane generally perpendicular to the axis of rotation of the metering rim . the lower guide ( 10 ) is formed in the shape of a partially open circular eyelet or pigtail . this allows strand leaving the surface of the metering rim to rotate freely inside the eyelet to accommodate the &# 34 ; balloon &# 34 ; effect of the strand as it is twisted by the combined action of the traveler ring ( 5 ) and rotating bobbin ( 4 ). ( this ballooning effect , well understood by those skilled in the art , describes the general shape of the strand due to the tension and centrifugal forces acting on it as it is wound onto the rapidly rotating bobbin .) this motion is also transferred to the beveled portion of the rim &# 39 ; s surface where the strand sweeps the surface left to right . the opposite ends of both the upper ( 9 ) and lower ( 10 ) strand positioning guides are inserted into holes located in one side of a separable housing ( 11 ), positioned , and then secured by means of a set screw , hex nut or other conventional threaded fastener ( 12 ) inserted at a right angle to guide . the exact location of guide bars may be further adjusted by bending them left or right so that the angle at which the strand ( 2 ) crosses the surface of the metering rim ( 6 ) may be further adjusted as illustrated in fig2 and 3 . the improved metering rim ( 6 ) which is further illustrated in fig4 is an annular member formed from a single piece of lightweight polyvinyl chloride or other moldable , low density , rigid , and smooth plastic material by either a machining or molding process . in the preferred embodiment , the rim has raised flanges ( 13 ) at each end which extend radially outward so that a track or raceway upon which the strand may ride is formed between them as illustrated in fig2 . turning again to fig4 the rim ( 6 ) is concentrically mounted over the upper annular portion of a stationary hub ( 14 ). the wall of the annulus contains a plurality of uniformly spaced bleed holes ( 15 ) of small diameter oriented toward the centerline . although the bleed holes extend through both the interior and exterior surfaces of the annular hub , they terminate slightly below its outside surface within a grooved cavity or cross - slot ( 16 ) formed in the outer wall . the purpose of these rectangularly shaped cavities or cross - slots will be explained in further detail below . the base of the hub ( 17 ) is in the shape of a solid circular disc having an outside radius greater than that of the upper annular portion . in the preferred embodiment , the entire hub was machined from a single piece of material although the use of a hub constructed from at least two separately manufactured parts , as well as a single molded unit requiring minimal fabrication or machining , is also contemplated . a nozzle ( 18 ) is provided through the base of the hub ( 17 ) for the injection of a substantially continuous supply of pressurized gas such as air . the nozzle can be any commercially available fitting used for injecting a pressurized source of gas . a hole drilled through the base of the hub and threaded so as to receive one end of the fitting is provided . a hose supplying pressurized gas from a supply source such as a remote compressor is then attached to the nozzle and the gas enters the interior of the annulus . this is not the only manner by which a pressurized gas can be supplied to the interior of the hub and an embodiment in which the hub and nozzle are formed as a single component is also contemplated . a round end cap ( 19 ) is provided along with means for securing it to the face of the upper annular portion of the hub ( 14 ), the end cap having substantially the same radius as the base of the hub ( 17 ). the end cap may be secured in place by conventional threaded fasteners ( 20 ) but the use of glue , adhesives , and heat bonding is also contemplated . a conically shaped strand threading guide ( 8 ) may also be attached by conventional fasteners to the end cap ( 19 ). this stationary threading guide has the general shape of frustum of a right conical shell of revolution and its smallest diameter is more or less equal to that of the end cap ( 19 ). the base of the threading guide may also be cupped as shown in fig2 so as to partially encompass one side of the end cap . in yet another embodiment , the use of a threading guide and end cap comprising a single molded or machined component is also contemplated . a conically shaped recess or surface is cut into one side of the housing ( 11 ) to accommodate the shape of the threading guide ( 8 ). the recess has a radius of curvature everywhere slightly greater than that of the strand threading guide . this creates a curved passage or guideway through which the strand ( 2 ) may be inserted and wrapped about the circumference of the metering rim ( 6 ). in the preferred embodiment , the outer surface of the annular metering rim ( 6 ) which comprises the track or raceway described previously is divided into at least two sections through its midplane as illustrated in fig2 . the metering surface ( 21 ) is of a constant outside diameter . the second surface ( 22 ) is beveled inwardly at an angle of anywhere from 0 ° to 10 ° from the horizontal , but preferably at about 5 °, so as to produce a tapered or conically shaped surface of gradually decreasing diameter . a plurality of slots or apertures ( 23 ) are then provided about the circumference of this second beveled surface as illustrated in fig2 and 4 . the slots ( 23 ) are generally of about the same width as the rectangular cross - slots ( 16 ) cavities cut into the outside surface of the hub ( 14 ) but extending only about half their same length . the pressurized gas or air bearing assembly comprising the hub ( 14 ), rim ( 6 ), and end cap ( 19 ) is positioned inside the front of the housing ( 24 ) as shown in fig4 and the other corresponding half ( 11 ) is positioned in place . a plurality of conventional threaded fasteners ( 25 ) are then used to secure the two halves of the housing together . another conventional threaded fastener ( 26 ) is then inserted through one side of the housing ( 24 ) and tightened so as to engage the base of the hub ( 17 ) and thereby hold the bearing assembly and attached threading guide ( 8 ) securely in place . in the preferred embodiment , the operation of the assembled apparatus is accomplished by connecting a source of compressed air having a gauge pressure of between 35 and 90 psi to the nozzle ( 18 ). due to the presence of the end cap ( 19 ), the air escapes radially outward through the bleed holes ( 15 ) in the wall of the annular portion of the hub ( 14 ) whereupon it fills the rectangular cavities or cross - slots ( 16 ) mentioned previously . this distributes the pressurized air more evenly across the width of the hub as it pushes against the inside surface of the annular metering rim ( 6 ). in the preferred embodiment , two rows of eight uniformly spaced bleed holes were used so as to introduce the pressurized air on alternating sides of the 16 corresponding cavities as depicted in fig4 . the pressure of the escaping air supports the rim ( 6 ) away from the hub ( 14 ) and maintains it in a substantially concentric position about it . the moving strand coming in contact with the rim causes it to rotate with substantially zero friction and in a substantially concentric manner about the hub . should the rim become displaced from concentric rotation , a restoring force will be generated due to the creation of a small imbalance in the circumferential pressure distribution of the layer of air supporting the rim . this pressure imbalance will increase as the rim is displaced further and further from a concentric position about the hub . as it does , so too will be the restorative force which acts to push the rim back into position . thus , the combined action of the pressure imbalance and restorative force continually acts to maintain the position of the rim . as the slots ( 23 ) located in the beveled surface of the rim pass directly over the rectangular cross - slots ( 16 ) in the hub ( 14 ), a short burst of air will pass through the surface ( 22 ). this air then sloughs off any broken filaments of strand from the metering rim ( 16 ) by casting them away generally in the direction of the taper of the beveled surface ( 22 ). in the preferred embodiment , the bleed holes ( 15 ) are very small , typically on the order of 0 . 018 inches in diameter and the cavities or cross - slots ( 16 ) are on the order of about 0 . 03125 inches wide and deep and about 0 . 25 inches long . a rim constructed of polyvinyl chloride having a density of approximately 0 . 04 lb / in 3 weighing about 4 . 3 grams was used . the rim itself had has an inside diameter of 1 . 750 inches and an outside diameter of 1 . 820 inches at the constant diameter metering surface ( 21 ) which comes in contact with the moving strand ( 2 ). neglecting the effect of the flanges ( 13 ) and slots ( 23 ), a rim of this size has a moment of inertia about its axis on the order of about 2 . 0 × 10 - 6 lb - ft - sec 2 . a strand moving at a typical lineal velocity of between 600 and 700 feet per minute and contacting the rim will cause it to rotate at an angular velocity between 137 and 160 radians per second or roughly 1300 to 1500 rpm . in order to count the number of revolutions of the rim , there is bonded at one location on the constant diameter metering surface ( 21 ) a light reflective target ( 27 ) in the shape of a dot or stripe made from a material such as thin aluminum foil . both a light emitting diode ( 28a ) and phototransistor ( 28b ), the operation of which will be described below , are also contained inside the housing and mounted along with a printed circuit board ( 29 ) carrying various electrical components , also described below , by a conventional threaded fastener ( 30 ). at least two other conventional threaded fasteners ( 31 ) are provided to secure the printed circuit board ( 29 ) and its associated electrical components to one - half of the separable housing ( 24 ). the housing is of such construction that it is capable of receiving these threaded fasteners at appropriately bored and tapped locations ( 32 ). the housing further contains an access hole or port ( 33 ) through which to pass a plurality of electrical wires or cables ( 34 ) carrying electrical signals used to turn off or on the drive motors of the twist - frame , ply frame , winder , or other powered winding mechanism used to advance the strand . light emitted from the diode ( 28a ) is reflected by the target ( 27 ) carried on the metering surface ( 21 ). this reflected light is then sensed by the phototransistor ( 28b ) whereupon a pulse of electrical current is generated and converted into a voltage once during each revolution of the rim . this voltage is then used to trigger a monostable timing circuit which produces a second voltage pulse having a known magnitude and duration that is compatible with the signal requirements for existing microcomputers . in the preferred embodiment , the light emitting diode and phototransistor are combined and housed in a single commercially available unit such as model 0pb - 703a , manufactured by trw corporation . a commercially available timing circuit such as type ne555 , manufactured by signetics corporation , and a microcomputer such as type 8751h , manufactured by intel corporation , may be used although other equivalent components may be substituted to accomplish substantially the same results . the microcomputer counts the number of voltage pulses which , in turn , correspond to the number of revolutions of the rim . the microcomputer may be further programmed to calculate the total strand length by multiplying this number by the circumference of the metering surface ( 21 ). as the rim rotates , these pulses will occur with a periodicity corresponding to the frequency of rotation of the rim . the microcomputer may also be programmed to activate a relay circuit in order to interrupt any of the powered winding mechanisms described previously once a target count or preset strand length is achieved . the target count , actual count , and other parameters such as the diameter or circumference of the metering surface may be programmed and transmitted to and from the microcomputer by any commercially available serial communications interface device such as a max232 device manufactured by maxim corporation . several alternative embodiments of the instant invention have now been disclosed in which certain specific aspects of the apparatus have been described . it should be understood that the instant invention is not limited to the use of any of these specific materials , electrical devices , or dimensions whenever others may be substituted to accomplish substantially the same results in substantially the same manner . although the preferred embodiment contemplates the use of this apparatus and method for measuring glass fiber strand in a twist - frame environment , it will be understood that it is not limited exclusively thereto . for example , the source for supplying essentially continuous lengths of strand to a take - up reel such as a bobbin or forming package may encompass previously made forming packages of strand or yarn already wound onto bobbins and then re - twisted onto other bobbins by a ply - frame . the source may also be strand drawn and attenuated from either a direct - melt or marble - melt fiber glass bushing assembly . furthermore , the measuring apparatus described herein may be used with any other type of natural or synthetic strand and is not limited to glass . therefore , the particular components and applications of the apparatus described herein are not to be interpreted as limiting except ; insofar as recited in my accompanying claims .