a self - cleaning lower contact 100 constructed in accordance with a preferred embodiment of the present invention may be used to perform electrical measurements of miniature electrical circuit components , or devices - under - test ( duts ) 14 , in a high - speed measurement system such as a model 3340 multi - function tester , manufactured by electro scientific industries , inc . of portland , oreg . fig5 a , 5 b , 5 c , and 6 show the general construction of lower contact 100 and its operational relationship with an upper contact 102 . lower contact 100 is spatially aligned with a corresponding upper contact 102 . duts 14 are seated in test plate 5 , which is positioned between lower contact 100 and upper contact 102 . lower contact 100 is formed of an electrically insulating cylindrical housing 104 having a substantially planar upper end surface 106 and a substantially planar lower end surface 108 . an extensible contact element 110 is movable in an interior opening 112 of housing 104 and outwardly of upper surface 106 . interior opening 112 is preferably a centrally located slot of generally rectangular cross section extending along the entire length of housing 104 . contact element 110 has an end portion 114 that terminates in a contact tip 116 , which contacts a terminal 118 of dut 14 as it undergoes a test process . the interior walls defining lengthwise slot 112 within housing 104 provide bearing surfaces for end portion 114 against which it moves to support straight line motion of contact element 110 along a longitudinal axis 120 of housing 104 . a biasing mechanism 130 , including , for example , spring - loaded pin (“ pogo pin ”) 44 , applies a force to the end of contact element 110 positioned farther from terminal 118 of dut 14 . biasing mechanism 130 urges contact tip 116 against terminal 118 of dut 14 as it undergoes the test process and against a lower surface 134 of test plate 5 as it transports dut 14 to and from different corresponding pairs of upper and lower contacts 116 and 118 . upper contact 102 is composed of a resilient flat metal cantilevered arm 136 with one end secured to a support module ( not shown ) and the opposite end terminating in a spring - biased roller contact 140 that contacts a terminal 138 of dut 14 . upper contact 116 is of a roller wheel type described in u . s . pat . no . 6 , 040 , 705 , which is assigned to the assignee of this patent application . when test plate 5 advances to place dut 14 in physical contact with contact tip 116 of lower contact 100 , lower surface 134 of test plate 5 slides over at least a portion of contact tip 116 , depressing it so that it is flush with upper surface 106 of housing 104 . the upward force holds contact tip 116 in physical contact with lower surface 134 of test plate 5 and terminal 118 of dut 14 during testing . when testing is complete , the position of test plate 5 is advanced ( to present the next dut 14 to lower and upper contacts 100 and 102 for testing ) and the upward force causes contact tip 116 to rub against lower surface 134 of test plate 5 . thus , contaminants or oxides are rubbed off of contact tip 116 , so that it is self - cleaned during operation of electrical component handler 2 , thereby eliminating the need for costly and time - consuming shutdown of the electrical component handler to clean lower contact 100 . contact tip 116 undergoes wear removal of contact tip material that results in progressive shortening of contact element 110 as test plate 5 transports dut . end portion 114 of contact element 110 is preferably in the shape of a flat blade , and contact tip 116 may be formed by automated stamping of a photochemical or highly conductive material , such as solid coin silver ( 90 wgt % ag , 10 wgt % cu ) or plain , soft copper . contact element 110 and contact tip 116 are preferably made of the same material , but contact tip 116 could be formed by depositing material having suitable wear characteristics on end portion 114 of different material . contact tip 116 has a usable depth of about 3 . 8 mm ( 0 . 15 in ) and has a sloped leading edge 144 to ensure terminal 118 of dut 14 does not catch on contact tip 116 as test plate 5 advances dut 14 to its measurement position between lower and upper contacts 100 and 102 . a conductive coating , such as gold , silver , or rhodium , may be applied to contact tip 116 to increase its resistance to oxidation during storage . to prevent the upward force from ejecting dut 14 from test plate 5 , a downward force that is greater than the upward force may be simultaneously exerted on or by test plate 5 against lower contact 100 . the downward force may be exerted by , for example , vacuum plate 32 , a plate retainer roller 146 , or both of them . further , contact tip 116 is preferably wider than seats 4 in test plate 5 such that the upward force applied against lower contact 100 does not eject dut 14 upward and out of its seat 4 . fig6 shows housing 104 of lower contact 100 extending through a hole in vacuum plate 32 such that housing 104 is positioned adjacent lower surface 134 of test plate 5 . a preferred vacuum plate 32 is in the form of a disk - like annulus and includes vacuum channels that are concentrically adjacent to seat rings 3 . vacuum channels 34 are connected to a partial vacuum pressure source ( not shown ) and communicate partial vacuum pressure to seats 4 of each ring 3 by way of linking channels through the thickness dimension of test plate 5 at seats 4 . the partial vacuum pressure created by vacuum plate 32 holds it adjacent to lower surface 134 of test plate 5 and dut 14 . this positioning results in contact tip 116 simultaneously being in contact with dut 14 and lower surface 134 of test plate 5 during a test cycle . the partial vacuum pressure draws through vacuum channels 34 contact tip material dust produced by wear of contact tip 116 . residual amounts of the dust accumulate on the bottom surface of test plate 5 , which is periodically cleaned to remove the dust . recent technological advances in component miniaturization have resulted in the formation of duts 14 having length and width dimensions of about 0 . 016 × 0 . 008 × 0 . 008 ( 0402 “ metric ” components ) and 0 . 024 × 0 . 012 × 0 . 12 ( 0201 “ metric ” components ). because of their small size , these duts are unable to tolerate large gaps , ledges , or interruptions in the surface geometry or channels along which they are transported during the testing process . if they encounter gaps , ledges , or interruptions , duts 14 can be easily damaged , since their terminal ( electrode ) ends are often coated with soft tin plating . thus , in a preferred embodiment of electrical component handler 2 , dut 14 passes over only a very small clearance gap . for example , in one preferred embodiment , contact tip 116 is wider than the device - receiving seats 4 in test plate 5 . this geometry also prevents the edge of contact tip 116 from interfering with a side wall of one of the device - receiving seats 4 in test plate 5 . electrically insulating housing 104 is preferably constructed of either ceramic - or plastic - based wear - resistant material . plastic material includes polycarbonate or polyphenylene sulfide ( pps ), and ceramic material includes transition - toughened zirconia ceramic or alumina . housing 104 is preferably designed to be easily removed during periodic machine maintenance and to facilitate changing vacuum plate 32 . electrically insulating housing 104 is preferably aligned relative to concentric rings 3 of component seats 4 such that upper surface 106 tracks the path of advancement of dut 14 . this alignment can be achieved by incorporating aligning features on electrically insulating housing 104 , by using an alignment tool , or by a combination thereof . various embodiments of lower contact 100 are in the form of multiple - component assemblies and are described below and shown in the drawing figures associated with their descriptions . fig7 a shows lower contact 100 having a housing 104 of single - piece construction of molded or cast electrically insulating material . fig7 b and 8 show different embodiments of a double - sided contact element 110 set in slot 112 of housing 104 of single - piece construction . double - sided contact element 110 is advantageous because the presence of two contact tips 116 prolongs its useful life . double - sided contact element 116 has two end portions 114 , each terminating in a contact tip 116 . in the embodiment of fig7 b , end portions 114 are connected on opposite sides by a sinuous resilient contact body portion 150 that does not contact the interior housing walls defining slot 112 . resilient contact body portion 150 is made of phosphor bronze or beryllium copper for resilience . end portions 114 and contact tips 116 may also be made of these materials to provide a one - piece resilient contact element 110 . end portion surfaces 152 move along bearing surfaces 154 of the interior housing walls defining slot 112 in response to force applied by biasing mechanism 130 . in the embodiment of fig7 b , resilient body portion 150 provides the elasticity in the biasing force and , therefore , constitutes a part of biasing mechanism 130 . in the embodiment of fig8 , end portions 114 constitute opposite distal ends of a unitary contact body portion 156 of generally rectangular cross - section , the end surfaces 152 of which extend along the length of contact element 110 and thereby continuously contact bearing surfaces 154 . contact element 110 has a length defined as the distance measured between contact tips 116 along longitudinal axis 120 , and housing 104 has a length defined as the distance between upper surface 106 and lower surface 108 . the length of contact element 110 ( which becomes progressively shorter over time with wear ) is greater than the length of housing 104 to enable contact tip 116 in proximal position to a dut 14 to contact its electrode and to enable biasing mechanism 130 to engage contact tip 116 in proximal position to its end 132 . fig9 a and 9b show housing 104 of single - piece construction in which a contact element 110 of generally rectangular cross - section such as that shown in fig8 but with a slot 170 extending along the length of contact element 110 . housing 104 has a hole 172 through which a screw 174 or other suitable fastener passes to secure contact element 110 in place and that is suitably positioned to limit to the slot length the travel of contact element 110 . fig1 a , 10 b , and 10 c show housing 104 of multiple - piece construction of electrically insulating material composed of two body components in the form of generally cylindrical segments 176 and 178 . cylindrical segments 176 and 178 have complementary mating surfaces that fit together to form slot 112 . in one embodiment , cylindrical segments 176 and 178 have axially aligned apertures through which screw 174 passes to assemble housing 104 and hold in place slotted contact element 110 in the same manner as that shown in fig9 a and 9b for housing 104 of single - piece construction . in another embodiment , cylindrical segments 176 and 178 are glued together . in both of these embodiments , the opposing inner walls of cylindrical segments 176 and 178 defining slot 112 function as bearing surfaces 154 along which contact element 110 slides . skilled persons will appreciate from the foregoing descriptions that housing 104 of either single - piece or multiple - piece construction may include a slotted or a nonslotted contact element 110 , a contact element 110 with a single - sided or double - sided contact tip 116 , or interior bearing surfaces provided by a lining material for slot 112 or the slot - defining interior surfaces of housing 104 itself . fig9 a and 9b and fig1 a , 10 b , and 10 c show a chamfer 180 formed in upper surface 106 of housing 104 of single - piece construction and multiple - piece construction , respectively . chamfer 180 provides for dut 14 a slide ramp of positive slope as test plate 5 moves dut 14 off of lower contact 100 upon completion of a test measurement . this ensures that dut 14 will not catch on an edge of slot 112 as dut 14 is moved away from lower contact 100 . fig1 a , 11 b , and 11 c show an embodiment of lower contact 100 for use in four - terminal kelvin - connection measurements . the kelvin contact embodiment of lower terminal 100 includes two lower terminal contact elements 110 1 and 110 2 that are separated within housing 104 by an electrical insulator 190 . terminal contact elements 110 1 and 110 2 are angularly inclined within housing 104 relative to longitudinal axis 120 , such that their respective contact tips 116 1 and 116 2 converge at upper surface 106 of housing 104 . for housing 104 of single - piece construction , insulator 190 is an integral part of the interior of housing 104 , in which terminal contact elements 110 1 and 110 2 move along angularly inclined slots formed in housing 104 . for housing 104 of multiple - piece construction , insulator 190 could be either a separate internal spacer component positioned between terminal contact elements 110 1 and 110 2 when housing 104 is assembled or an integral part of one of the housing body components . biasing mechanism 130 comprises separate spring - loaded pin ( pogo pin ) contacts 44 1 and 44 2 that apply force against the respective ends of terminal contact elements 110 1 and 110 2 proximal to lower surface 108 of housing 104 to urge contact tips 116 1 and 116 2 against different regions of terminal 118 of dut 14 . fig1 shows a single - piece common housing 104 m in the form of a rectangle with rounded edges and four slots 112 spaced apart to receive multiple separate contact elements 110 . contact elements 110 may be of either single - sided or double - sided type . the interior construction of slots 112 is similar to that of housing 104 shown in fig7 a , 7 b , and 8 . fig1 a shows a vacuum plate 32 m , which is vacuum plate 32 modified to accommodate common housing 104 m . vacuum plate 32 m is formed by removing the material separating holes 30 to form an elongated slot 30 m sized to receive housing 104 m . fig1 b shows a base member 36 m , which is base member 36 modified to clamp common housing 104 m between upwardly projecting wall 38 and releasable clamping mechanism 42 . base member 36 m is formed by removing the scallop segments 40 of base member 36 to form a smooth surface recess 40 m into which common housing 104 m can fit securely when clamped . fig1 a and 14b show an optical contact wear sensor 200 that detects when contact tip 116 is worn down and therefore contact element 110 needs either replacement or , if double - sided with an unused side available , reversed in direction of movement of housing 104 . contact wear sensor 200 includes a light emitter 202 and a light detector 204 positioned on either side of contact element 110 protruding from lower surface 108 of housing 104 and arranged to provide a direct line of sight light propagation path . as shown in fig1 a , whenever contact tip 116 has sufficient electrode material remaining , contact element 110 interrupts the light propagation path between light emitter 202 and light detector 204 . as shown in fig1 b , whenever contact tip 116 has undergone an amount of electrode material removal that shortens the length of contact element 110 by a predetermined amount , light propagating from light emitter 202 reaches light detector 204 to produce a signal indicating excessive wear of contact tip 116 . a contact head designed in accordance with the present invention was tested in a model 3300b multi - function tester . the test conditions were as follows : the dut was a 100 pf capacitor , the test frequency was set to 1 mhz , the upper limit for the decontamination factor ( df ) was set at 0 . 0007 , and the dut actual df was set to between about 0 . 0002 and about 0 . 0003 . the results of the test were as follows : of the 113 , 257 duts that were tested , only 0 . 51 % of them were falsely rejected . in comparison to the about 5 . 89 % false rejection rate of prior art contact heads , this is a significant decrease in false rejections . this decrease in false rejection results in a consequent increase in productivity and machine throughput . it will be obvious to those having skill in the art that many changes may be made to the details of the above - described embodiments without departing from the underlying principles of the invention . for example , preferred embodiments of the contact head of the present invention include a plurality of lower contacts for simultaneously testing more than one dut , but can also include a single lower contact . the scope of the present invention should , therefore , be determined only by the following claims .