referring now to the drawings , fig1 shows a plan view of a prior art disc drive 100 in which the present invention is particularly useful . the disc drive 100 includes a base member 102 to which all other components are directly or indirectly mounted and a top cover 104 ( shown in partial cutaway ) which , together with the base member 102 , forms a disc drive housing which encloses delicate internal components and isolates these components from external contaminants . as discussed in greater detail below , the disc drive includes a plurality of discs 106 which can be advantageously evaluated for surface defects prior to incorporation into the disc drive 100 . the discs 106 are mounted for rotation on a spindle motor shown generally at 108 . the discs 106 include on their surfaces a plurality of circular , concentric data tracks , the innermost and outermost of which are shown by dashed lines at 110 , on which data are recorded via an array of vertically aligned head assemblies ( one of which is shown at 112 ). the head assemblies 112 are supported by head suspensions , or flexures 114 , which are attached to actuator head mounting arms 116 . the actuator head mounting arms 116 are integral to an actuator bearing housing 118 which is mounted via an array of precision ball bearing assemblies ( not designated ) for rotation about a pivot shaft 120 . it will be understood that the head assemblies 112 preferably come to rest on texturized landing zones ( not designated ) at the inner radii of the discs 106 ( beyond the data recording surface region bounded by dashed lines 110 ) when the disc drive is deactivated . power to drive the actuator bearing housing 118 in its rotation about the pivot shaft 120 is provided by a voice coil motor ( vcm ) shown generally at 122 . the vcm 122 consists of a coil ( not separately designated ) which is supported by the actuator bearing housing 118 within the magnetic field of an array of permanent magnets ( also not separately designated ) which are fixedly mounted to the base member 102 , all in a manner well known in the industry . electronic circuitry ( partially shown at 124 , generally , and partially carried on a printed circuit board ( not shown )) to control all aspects of the operation of the disc drive 100 is provided , with control signals to drive the vcm 122 , as well as data signals to and from the heads 112 , carried between the electronic circuitry 124 and the moving actuator assembly via a flexible printed circuit cable ( pcc ) 126 . it will be apparent to one of skill in the art that the proper operation of the disc drive 100 will depend in large part to the existence of a controlled , precise relationship between the head assemblies 112 and the discs 106 . therefore , it is common in the industry to test each of the discs 106 included in the disc drive 100 before the discs 106 are assembled into a disc drive 100 . [ 0036 ] fig2 is a simplified functional block diagram of a typical prior art test system used to test and map the surface of recording discs as components before the discs are assembled into disc drives . the test system 130 includes a precision spin stand 132 which further includes a spin motor 134 on which the disc 106 is mounted for rotation and testing . the test system 130 also typically includes a linear actuator 136 which is used to controllably move a test head 138 , mounted on a head suspension 140 , on a linear path across a radius of the disc 106 . actuator control logic 142 is also included in the test system 130 and provides the control signals on signal path 144 needed to move the test head 138 and monitors , via signal path 146 , the position of the test head 138 during testing of the disc 106 . in a typical test system of the current art , the actuator supports and controls a second test head for simultaneous testing of the second disc surface . for purposes of clarity , fig2 only a single test head 138 . the test system 130 also includes spin motor control logic 148 which is used to accelerate the spin motor 134 to its intended testing speed . it is common practice in the industry to vary the speed of the spin motor 134 as the test head 138 is moved across the disc radius to provide a constant linear velocity between the test head 138 and the area of the disc being tested . that is , as the test head 138 is moved inward , the speed of the spin motor is increased proportionally to maintain a constant linear velocity , and thus maintain a constant flying height for the test head 138 . the spin stand 132 also includes a spin motor position detection element 150 which provides a “ once around ” reference signal . this reference signal is carried over signal path 152 to the spin motor control logic 148 where it is used to assist in the control of the speed of the spin motor 134 . the reference signal is also passed via signal path 154 to defect mapping logic 156 , where it is utilized , along with the actuator position signal passed via signal path 158 by the actuator control logic 142 , to maintain a constant calculation of the radial and circumferential portion of the disc 106 that is located under the test head 138 . during the testing operation , a disc 106 is mounted on the spin motor 134 and the spin motor 134 is brought up to operational speed by the spin motor control logic 148 . once the spin motor 134 is at the proper speed , the actuator control logic 142 causes the actuator 136 to move the test head 138 into cooperative arrangement with the surface of the disc 106 . the test head 138 is then stepped across the spinning disc 106 at a rate selected to cause the test head 138 to pass over every portion of the disc surface . as the head is stepped across the disc surface , the spin motor control logic 148 varies the spin motor speed to maintain a constant relative linear velocity between the test head 138 and the disc area being tested as noted above . a defect on the disc surface will cause the test head 138 to generate a defect signal which is passed to the defect mapping logic 156 via signal path 159 . recognition of the defect signal by the defect mapping logic 156 results in the current radial and circumferential location of the test head 138 relative to the disc 106 being recorded . once the test head 138 has passed over the entire usable radial extent of the disc 106 , all detected and recorded defects are correlated to produce a defect map of the entire disc surface . test systems of the type described above and which can be modified to include and implement the present invention are available from several sources . for example , a representative test system of this type is the model number msa 450 , manufactured by cambrian systems , inc ., a subsidiary of phase metrics corporation , located in westlake village , calif . [ 0043 ] fig3 and 4 are , respectively , simplified bottom and top perspective views of a typical prior art glide test head 160 . the glide test head consists of a slider body 162 which is typically formed from a stable ceramic material , such as aluminum oxide / titanium carbide . features of the slider body 162 are commonly formed using the processes of machining , ion etching and precision lapping . the glide test head 160 is of the type sometimes referred to as a “ catamaran ” or “ positive - pressure ” slider configuration , and includes a pair of laterally displaced rails 164 with air bearing surfaces 166 which interact with a thin layer of air dragged along by the spinning disc to fly the glide test head 160 at a desired fly height above the surface of the disc being tested . as is known to those of skill in the art , the flying height is determined , in part , by the geometry of the air bearing surfaces , and the flying attitude of the slider body is a function of the geometry of the air bearing surface , as well as the head suspension ( 140 in fig2 ) used to support the glide test head 160 . at the leading edge of the air bearing surfaces 166 the rails 164 also typically include beveled regions 168 which are included to aid in the rapid establishment of the air bearing between the slider body 162 and the spinning disc . [ 0046 ] fig3 and 4 show that the slider body 162 also includes a laterally extending wing 170 which is used to mount a contact sensor 172 , such as a piezoelectric crystal or element . the reason that the slider body 162 includes the wing 170 for mounting the contact sensor 172 is that portion ( shown at 174 , generally , in fig4 ) of the slider body 162 above the rails 164 is used to attach the head suspension ( 140 in fig2 ) used to support the slide test head 160 . the contact sensor 172 include attached signal wires 176 . during operation , such as in a test system similar to that of fig2 contact between the air bearing surfaces 166 and a surface asperity on the disc under test will result in oscillation or ringing of the entire slider body 162 . this excitation of the slider body 162 is conveyed to the contact sensor 172 which responds to this excitation by outputting electrical signals on the signal wires 176 . these electrical signals are passed to appropriate detection logic ( such as the defect mapping logic 156 of fig2 ). if , as noted in the discussion of fig2 above , the occurrence of the output of the contact sensor 172 is correlated to the position of the actuator and the rotational position of the disc under the glide test head , a defect map of the disc under test can be generated . [ 0048 ] fig5 provides a bottom perspective view of a glide test head 200 having a negative - pressure air bearing slider configuration constructed and operated in accordance with preferred embodiments of the present invention . for clarity , a leading edge of the glide test head 200 is generally identified at 202 . the glide test head includes a slider body 204 from which a plurality of rails 206 extend , the rails 204 having corresponding bearing surfaces 208 . during operation , air is diverted through channels ( not designated ) between adjacent rails 206 , forming regions of low pressure 210 which maintain the glide test head 200 in a stable relationship with the corresponding disc 106 . in a manner similar to that discussed above for the glide test head 160 of fig3 and 4 , the glide test head 200 of fig5 further includes a laterally extending wing 212 which supports a contact sensor 214 , having a piezoelectric or similar construction . as before , contact between the glide test head 200 and a feature of the disc under evaluation will result in the output of a signal via paths 216 . it will be noted that the operational characteristics of the negative - pressure glide test head 200 of fig5 will typically be more stable and controlled than the operational characteristics of the positive - pressure glide test head 160 of fig3 - 4 . for example , test results indicate that a three - sigma variation in fly height for a positive - pressure glide test head such as 160 can be as high as 113 a ( 0 . 45 μin ), whereas the three - sigma variation in fly height for a negative - pressure glide test head such as 200 can be less than 40 a ( 0 . 16 μin ). moreover , negative - pressure glide test heads have fly heights that are far less sensitive to rotational speed of the corresponding disc than positive - pressure glide test heads . the increased “ stiffness ” of the air bearing established by a negative - pressure glide test head provides increased signal resolution ( i . e ., higher signal - to - noise ratio ) in the output of the contact sensor , improving defect detection efforts . finally , since most read / write heads ( such as 118 , fig1 ) of the current generation of disc drives typically employ negative - pressure slider configurations , the use of a negative pressure slider configuration in a glide test head provides better correlation between the glide test heads and production read / write heads . however , the very advantages which make negative - pressure slider configurations desirable for use in production read / write heads also tend to be disadvantageous when attempting to incorporate such configurations in glide test systems . unlike prior art positive - pressure slider configurations , the fly height of which can be readily controlled by simply speeding up or slowing down the linear velocity of the disc , negative - pressure slider configurations tend to fly in a much more stable manner at a nominal height above the disc and are accordingly less affected by disc rotational speed . as discussed above , glide testing is typically performed at a fly height that is substantially lower ( such as 50 % lower ) than the nominal operational fly height of production read / write heads in order to evaluate surface height defects ( such as localized “ peaks ” on the discs ). for example , a glide test system may test at a nominal fly height of 125 a ( 13 nm or 0 . 5 μin ) over data portions of the disc for a corresponding nominal operational fly height for production read / write heads of 250 a ( 25 nm or 1 . 0 μin ). at the same tame , landing zone portions of the disc , also desirably evaluated during glide testing , may have texturized peaks with heights on the order of 165 a ( 17 nm or 0 . 66 μin ). thus , heretofore it has been difficult in the prior art to successfully incorporate negative - pressure slider configurations into glide test systems . [ 0055 ] fig6 provides a schematic diagram of a portion of a glide test system 220 constructed and operated in accordance with preferred embodiments of the present invention . it will be understood that the glide test system 200 generally includes various portions of the prior art glide test system of fig2 and so for purposes of clarity such have not been replicated in fig6 . [ 0056 ] fig6 shows the glide test head 200 of fig5 in conjunction with an evaluation disc 106 and a voltage source 222 which controls the fly height of the glide test head 200 with respect to the disc 200 in relation to a magnitude of a fly height control voltage impressed across the glide test head 200 and the voltage source 222 . aspect ratios are exaggerated for clarity . more particularly , as discussed more fully in co - pending u . s . patent application ser . no . 09 / 258 , 091 filed feb . 24 , 1999 by feng et al ., assigned to the assignee of the present invention , the glide test head 200 and the disc 106 can be characterized as opposing plates of a variable capacitor , with at least the air layer supporting the glide test head 200 above the disc 106 comprising an intermediate dielectric layer . although not shown in fig6 it will be understood that disc 106 can further be supplied with various substrate layers , such as a layer of lubricant with a nominal thickness of 25 a ( 3 nm or 0 . 2 μin ), and these substrate layers can further contribute to the dielectric layer . this capacitor can be modeled as follows : where c is the capacitance of the system , d is the head / disc gap , a is the surface area , and e and e r are the absolute and relative permittivity of the dielectric layer , respectively . the attractive force between the glide test head 200 and the disc 106 can be expressed as follows . it can be seen from equation ( 2 ) that the head / disc gap , d , can be readily changed by varying the applied voltage v . for example , an increase in voltage v leads to a higher attractive force f , resulting in a corresponding reduction in the gap d . contrawise , a decrease in the voltage v leads to a lower attractive force f , increasing the gap d . it is further contemplated that the same polarity of charge might be applied to both the glide test head 200 and the disc 106 to establish a repelling force rather than an attractive force , as desired . accordingly , the glide test head 200 is configured to operate at a nominal fly height above the surface of the disc 106 , with the voltage source 222 operating to deflect the fly height of the glide test head 200 from this nominal fly height in relation to a magnitude of the control voltage applied thereby . the glide test head 200 can be charged either positively or negatively with respect to the disc 106 , and the voltage can either be a direct current ( dc ) voltage of selected magnitude or an alternating current ( ac ) voltage at selected frequency and magnitude . it will be understood that the control voltage is not applied to an active element of the head , as during normal data transducing operations ( that is , the control voltage is not the sane thing as a write current , for example ); rather the control voltage is applied to charge the slider body 204 with respect to the disc 106 ( irrespective of the actual polarities involved ). referring now to fig7 shown therein is a flow chart for an adaptive fly height defect mapping routine 230 , generally illustrating steps carried out by the glide test system 220 of fig6 in accordance with preferred embodiments . at step 232 , the particular disc 106 to be evaluated is first mounted in the glide test system 220 . after bringing the disc 106 up to a desired rotational speed , the glide test head 200 is next moved out over the disc surface , step 234 . it will be noted that at such time that the glide test head 200 aerodynamically engages the air currents established by the rotation of the disc 106 , the glide test head 200 will be supported over the disc 106 at a nominal flying height determined by the characteristics of the negative - pressure air bearing slider configuration thereof and , to a lesser extent , the rotational speed of the disc 106 . at step 236 , the desired control voltage is applied across the glide test head 200 and the disc 106 to adjust the fly height of the glide test head 200 to a desired height to evaluate the data region of the disc 106 . in one embodiment , empirical test data can be utilized to correlate desired fly height to applied control voltage for each glide test head ; alternatively , closed loop measurement techniques can be utilized to adjust the applied control voltage to maintain the fly height at a desired level . during the operation of step 236 , defects are detected and mapped as discussed above . continuing with the flow of fig7 the glide test system 220 next preferably adjusts the control voltage to select a fly height for the glide test head 200 suitable for use in evaluating the texturized landing zone region of the disc 106 . as discussed above , the surface features of the landing zone are typically substantially higher than those of the data region of the disc to eliminate stiction forces between the production read / write heads ( such as 112 in fig1 ) when such come to rest on the landing zones . if an unacceptable level of defects is identified during the operation of the routine of fig7 the flow passes from decision step 240 to step 242 wherein the disc is rejected and reworked as necessary . alternatively , if the disc surface characteristics are deemed acceptable , the routine passes from decision step 240 to step 244 where the disc is approved for incorporation into a disc drive such as 100 . the routine then ends at 246 . it will now be apparent that the present invention , as embodied herein and as claimed below , provides several important advantages over the prior art . the use of a glide test head with a negative - pressure air bearing slider configuration to provide a stable flight profile in combination with a voltage source to adaptively control the flying height of the glide test head facilitates evaluation of disc surface characteristics in an efficient and highly accurate manner , particularly for reduced glide test fly heights of 125 a ( 13 nm or 0 . 5 μin ) or less . moreover , greater detection sensitivity is afforded due to the improved stiffness of the glide test head air bearing , allowing more precise defect mapping and evaluation . it will be clear that the present invention is well adapted to attain the ends and advantages mentioned as well as those inherent therein . while presently preferred embodiments have been described for purposes of this disclosure , numerous changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the invention disclosed and as defined in the appended claims .