in general , the apparatus and method of this invention for analyzing a test specimen for gold content or other precious metal content is based on developing and measuring an electromotive force ( emf ) due to electrical conduction between two dissimilar metals or metal alloys , namely an electrode and the sample to be analyzed . u . s . pat . nos . 2 , 531 , 747 and 3 , 282 , 804 to stearn generally describe this type of analytical technique for analyzing metal alloys , and the patents are hereby incorporated . the galvanic voltage results from the difference in availability of electrons in the different metals or alloys , and provides a net current when the metals or alloys are coupled through an electrolyte . the galvanic voltage generated by the dissimilar metals or metal alloys is generated using the touch probe of the invention , and is measured by subtracting the galvanic emf from that of a battery or similar reference level at a constant reverse polarity voltage . the test circuit is adjusted to provide a full scale reading and the net galvanic output voltage measured as percentage of full scale . the measurement is roughly proportional to the gold or other precious metal content of the test specimen and provides a means by which the purity of the sample can be tested . this technique is potentially useful to determine the nature of the metal in the sample , but is primarily useful for determining the purity of precious metals such as gold . it is an aspect of the invention that such a measurement is made using an improved form of electrode probe . fig1 is a cross - sectional view of a first embodiment of the hand - held test probe of the invention . probe 1 generally comprises a cylindrical body 2 made of plastic or other substantially electrically non - conductive material . a top cap 3 on the probe 1 is fitted with a jack 4 to enable electrical connection of the probe to a lead or wire 5 coupled to the electrical circuitry for providing the reference voltage and making the emf measurement . the electrical connection can be made via a miniature phone jack and plug as shown . the end of probe 1 to be touched against the sample is fitted with an absorbent , non - conductive , acid resistant synthetic compressed fiber tip 6 . the tip is advantageously of the type used in fiber tipped markers , highlighters and the like , namely including a nonwoven batt of synthetic fibers . the fiber tip 6 is preferably formed with a truncated conical or tapered flat end 7 , the sides of which are coated with a non - conductive epoxy resin 8 , as best illustrated in the enlarged view in fig2 . the epoxy resin serves several purposes . it reduces frictional wear on the end 7 , prevents the evaporation or leakage of electrolyte from the fiber tip 6 and defines a small , specific and repeatable area of actual contact between the fiber tip 6 and the specimen 10 being tested . the electrolyte is carried in the fibers of the tip by capillary action , and forms a path for electrical conduction between the probe and the sample . the mobility of the electrolyte in the tip can be improved by forming the nonwoven fiber batt using substantially aligned fibers , for example extending substantially parallel to the longitudinal axis of the probe . the electrolyte can be a liquid acid solution as known in conventional galvanic testing techniques , but is stored and deployed according to the invention in the particular probe arrangement as shown . aqueous solutions including one or more of hydrochloric acid ( hcl ), nitric acid ( hno 3 ) and ammonium chloride ( nh 4 cl ) are possible , in each case dissociating in water to provide free ions for the galvanic reaction . the rear end of the fiber tip 6 extends into a plastic or glass reservoir 9 contained in the interior of the probe body 2 . the reservoir 9 contains the liquid acid solution that is fed to the tip of the probe and serves as an electrolyte during the testing of the test specimen 10 , shown in fig1 . the fiber tip 6 is provided with a thin platinum wire 11 ( e . g ., about 0 . 45 mm diameter ) that preferably is embedded in the fiber tip 6 and extends into the reservoir 9 . in fig1 a second thin platinum connecting wire 12 couples the platinum wire 11 to the jack 4 in the top cap 3 of the probe 1 . a thin plastic monofilament line or a thin plastic tube 18 is positioned between the outer periphery of the fiber tip 6 and the inner wall of the body 2 of the probe 1 and serves as an atmospheric vent to the reservoir 9 . the platinum wire or electrode can be arranged and coupled to the electrical connecting lead 5 in a number of ways , and examples are shown in fig1 - 4 , with the same reference numbers identifying the same elements throughout the drawings . in fig1 and 2 , the electrode wire extends through the body of tip 6 and into reservoir 9 . a second platinum wire extends from connecting lead 5 downwardly into the reservoir , residing adjacent the wire from the tip . this arrangement allows the tip and associated wire to be inserted into the probe , but does not have a direct electrical connection along the two platinum wires . electrical connection is achieved through the free ions of the electrolyte in reservoir 9 . in fig3 the platinum wire in the probe connects directly through to the connecting jack 4 . when attaching the female part of the jack ( i . e ., when pushing the jack into the end of the barrel ), some slack is introduced , but the connection is direct . this arrangement has the advantage that the electrical resistance of the probe is less because the conductive path is mostly through conductive wire rather than through the electrolyte . a minimal electrical resistance is preferable because this resistance is in series with the test circuit and tends to reduce sensitivity . in each case , the electrode extends into the electrolyte in reservoir 9 , and in fig1 - 3 the electrode extends nearly to the exposed end of the tip . however , as shown in fig4 the electrode can also be contained fully in the reservoir portion , with a longer gap between the distal end of the electrode wire and the end of the tip . in this case , the electrical connection to the sample is also made through the tip by the electrolyte . the embodiment of fig4 is more readily assembled in that attention is not required to position a wire protruding from the tip as in fig1 - 3 . the tip can be a flattened fibrous tip of the type used for highlighters and markers , as shown in fig5 a and 5b , preferably including the epoxy protective portion 8 , leaving only a small area of 1 - 2 mm width , for touching to the specimen . the reservoir 9 preferably comprises an acid resistant absorbent fiber mass , saturated with the liquid electrolyte the probe is sized and shaped to fit comfortably in the user &# 39 ; s hand , for example comprising a cylindrical shape of about 15 cm in length and 1 . 75 cm in diameter in the area of reservoir 9 . the invention is applicable to testing of metals generally , and is particularly apt for testing specimens 10 of gold alloys to determine their purity . for testing specimens of between 4 and 18 karat gold content , for example , reservoir 9 is filled with an electrolyte , for example a solution of about 6 % hydrochloric acid , the remainder being distilled water . for testing of test specimens 10 of gold alloys above 18 karat gold content and for testing platinum , palladium and silver , reservoir 9 can be filled with a solution of about 3 % nitric acid , 5 % hydrochloric acid and 4 % ammonium chloride , the remainder being distilled water . the reservoir 9 can be a prefilled cartridge that is disposed of when empty . another preferred electrolyte solution is 12 % ammonium chloride and 1 % nitric acid . the adsorbent , non - conductive , acid resistant fiber tip 6 of the probe 1 always holds a small amount of the acidic electrolyte from the reservoir 9 , which wets a small area of the specimen and provides the galvanic contact between the platinum cathode wire 11 and the test specimen 10 ( see fig2 ). there is no need to immerse the specimen 10 in a container filled with an electrolyte or to cover a large portion of the specimen with an acidic paste or gel , as required in known devices . as shown in the circuit diagram of fig6 the probe 1 , and in particular the thin platinum wire 11 imbedded in the fiber tip 6 , serves as the cathode . the test specimen 10 serves as the anode ( see fig2 ). during the testing procedure the specimen is connected on the anode or more - positive side to wire 5 &# 39 ; by an alligator clip or other suitable electrical connecting device . the object is to measure precisely the electromotive force developed by galvanic action between the dissimilar metals of the sample and the platinum wire contained in the probe , e . g ., extending to within a short distance from the extreme end of the tip ( e . g ., 3 mm ), or at least extending into proximity with the tip . this is accomplished by calibrating the tester to predetermined voltage conditions , touching the probe to the sample , and measuring the change from the predetermined conditions indicative of the galvanic action . three embodiments for the circuit used with the probe are shown schematically in fig6 - 8 , and show some variations that are possible according to the invention . in each case , the circuits are battery powered and include adjustment means for setting the series resistance coupled between the battery and the sample , and adjustment means associated with a volt meter whereby the sensitivity of the circuit is maximized by preliminarily setting the meter to read 100 % of full scale . the galvanic action produced by the sample and electrode is measured as the extent to which the meter reading is reduced when the sample is coupled into the circuit . the same reference numbers identify circuit elements that correspond in the drawings . in fig6 . 5 volt aa battery 15 is connected to apply a voltage to the sample , via a switch 16 , a series fixed resistor r3 and a potentiometer vr2 . the potentiometer is adjusted to calibrate the circuit and to obtain the correct karat indication . the karat indication is obtained from meter m1 , which is preliminarily set at 100 % of full scale by adjusting potentiometer vr1 . diodes d1 and d2 provide a forward biased voltage drop of 0 . 7v each , and are coupled in parallel with the sample and in series with resistor r5 . meter m1 essentially reads the voltage on resistor r5 , which is in parallel with resistor r4 and a portion of the resistance of potentiometer vr1 -- the adjustment of potentiometer vr1 increases or decreases the resistance in parallel with resistor r5 . potentiometer vr1 also changes the series resistance coupled to the meter , but the meter has a high input impedance , so this effect is not material . for example , the meter can be a micronta model 22 - 221 or model 22 - 212b . other meters are possible . further loading is provided by resistor r6 , in parallel with the sample . millivoltmeter m2 can be used to set potentiometer vr2 to obtain the calibration setting needed . according to the resistance values shown in fig6 the test circuit was calibrated as shown in table i . table i__________________________________________________________________________calibration of vr2 8 kt 10 kt 14 kt 18 kt 24 kt % readings readings readings readings readings full meter meter meter meter meter turns ohms scale mv % mv % mv % mv % mv % mv__________________________________________________________________________0 0 48 1049 8 689 13 737 46 883 70 955 78 976 3 200 52 1070 9 702 17 766 47 892 72 975 82 1005 6 400 60 1120 12 737 19 776 48 931 72 1013 82 1048 7 . 5 500 66 1161 14 772 20 823 50 960 72 1057 82 1089 9 600 71 1178 14 769 23 836 52 976 72 1061 82 1101 12 800 84 1246 21 842 27 882 60 1055 72 1110 82 1145 15 1000 98 1320 30 935 42 1000 61 1164 78 1197 84 1238__________________________________________________________________________ an alternative embodiment of the meter circuit is shown in fig7 wherein diodes d1 and d2 are members of a rectifier bridge , and in the diode leg the circuit is loaded only by potentiometer vr2 and resistor r4 . no serial resistance is inserted with meter m1 , which is coupled directly in series with the two diodes . this circuit has good sensitivity . the alternative meter circuit of fig8 has a 9v power supply , preferably also using a battery . potentiometer vr1 is used to calibrate the circuit such that the open circuit probe produces a full scale reading on meter m1 , which in this case does not have a separate control on the meter leg and therefore is less subject to calibration adjustments . however the circuit produces stable and repeatable results . having calibrated the tester , probe 1 is touched against the sample 10 , which is in turn coupled to the positive terminal of battery 15 through switch 16 . the sample and probe form a galvanic cell or a second battery . when placed in the circuit , this galvanic second battery reduces the voltage reading at the meter to an extent that depends on the extent of galvanic action , which is strongest when the sample metal is most dissimilar to the platinum electrode , pure gold being more similar to platinum than alloyed gold , which includes metals that are farther from platinum on the periodic table . by testing reference samples of known purity , for example over a range of karat values from 8 to 18 karats , a table of voltages at millivoltmeter 17 can be developed , and the voltage reading thereafter obtained from an unknown sample can be compared with the reference values to determine the karat rating of the sample . the tester effectively couples the drive battery 15 and the galvanic battery of the sample and probe , at reverse polarity . thus , the drive battery stimulates the galvanic action at the sample . the emf produced by the galvanic battery reduces the voltage across resistor r2 , by an amount determined by the extent of galvanic action and by the resistance setting of potentiometer vr2 . the voltage across resistor r2 is reduced to a greater extent where the sample is very different than the platinum of the probe ( e . g ., at low karat values ), and to a lesser extent where the sample is more similar to platinum ( e . g ., at high karat values ). the particular electrolyte composition chosen can affect the sensitivity of the tester and the extent to which a mark is made at the site of contact between the probe and the sample . dilute hydrochloric acid , for example a 4 . 0 to 8 . 0 % solution , can be used as an electrolyte . however , this electrolyte does not always penetrate tarnished surfaces or surfaces that may be filmed over . a 1 . 0 to 4 . 0 % solution of nitric acid can be added , which helps penetrate tarnish and eliminates the need for preliminary cleaning of the sample . a combination of nitric and hydrochloric acids is also possible . a preferred electrolyte comprises nitric and hydrochloric acids , and ammonium chloride . ammonium chloride ( nh 4 cl ) is an ammonium salt of a relatively strong acid . as a salt , ammonium chloride is the combination of equivalent quantities of acid and base . it is readily soluble in water and accordingly dissociates in water to nh 4 + and cl - , forming an electrolyte solution having electrical conductivity . if replacement is complete , the salt is normal . however , if replacement is not complete , then some hydrolysis may occur where either the nh 4 + cation or the cl - anion in solution combines with either an oh - anion or an h + cation from the water , respectively , leaving an excess amount of the other ion in solution . this makes the solution either slightly acidic or slightly basic due to hydrolysis . ammonium chloride will become acidic in aqueous solution due to hydrolysis and loss of nh 4 + in solution . however , it provides additional free ions for carrying charge , and can increase the span of the test circuit and its sensitivity in distinguishing levels of gold purity . ammonium chloride in a 10 to 15 % solution can be used as the electrolyte , or preferably is combined with dilute nitric acid , which combination has been found to perform well in the tester of the invention and to improve the span ( sensitivity ) of measurement by 25 to 30 %. table ii represents a comparison of eight electrolyte formulations containing nitric acid , hydrochloric acid and / or ammonium chloride in a range of 0 - 4 % nitric , 0 - 8 % hydrochloric and 0 - 15 % ammonium chloride , these ranges being the weight proportions used mixing 70 % nitric acid solution by weight , with 38 % hydrochloric acid solution by weight , and solid ammonium chloride salt by weight . the meter readings shown are percent of full scale after calibrating the circuit of fig6 to 1 , 304 mv and a meter reading of 95 % of full scale . table ii__________________________________________________________________________electrolyte composition meter readings , % of full scale % hno . sub . 3 % hcl % hn . sub . 4 cl 8 kt 10 kt 14 kt 18 kt 24 kt pd pt__________________________________________________________________________0 6 0 42 - 44 46 - 50 68 - 72 80 93 95 - 98 98 3 5 4 32 36 60 75 92 90 - 96 98 4 8 4 16 22 44 60 74 70 - 80 88 0 0 15 28 32 50 62 70 70 - 88 94 1 0 12 39 42 64 74 84 86 - 94 97 0 8 0 28 - 32 36 - 44 58 - 62 72 - 76 80 - 82 86 - 88 94 2 4 3 28 34 60 76 87 86 - 92 98 1 0 10 36 40 64 78 88 88 - 90 96__________________________________________________________________________ fig4 - 7 are graphs showing test results obtained using the apparatus and methods of the invention , and comparing the readings obtained for different electrolytes . the slope of the curve representing meter reading vs . karat value varies over the karat range . when testing samples believed to fall within a particular range of karat values , the electrolyte and potentiometer setting can be chosen to provide the greatest difference ( i . e ., the highest slope ) in the range of interest . fig9 compares meter readings over a range of karat value vs . meter reading for two electrolytes , namely a weak acid mix of 1 %- 0 %- 12 % nitric , hydrochloric and ammonium chloride , and a substantially normal electrolyte of 15 % ammonium chloride solution . these results suggest that the weak acid mix is preferable at high karat levels , but either solution produces an easily distinguished span over the range of 8 - 18 karats . fig1 shows that a strong acid mix of 3 %- 5 %- 4 % nitric , hydrochloric and ammonium chloride , produces a very good span of readings over 6 - 24 karats . however , as discussed above , a strong acid solution is more likely to mark the sample . the millivolt readings obtained according to the circuit shown tend to rise over time if the tester remains connected to the sample , which is believed to be due to exhaustion of the available electrolyte at the sample . this effect is greater at low karat values than at high values , due to the greater galvanic action . for example , the reading obtained with a ten karat sample can rise several percent over one minute , while the reading for a 24 karat sample may rise only one percent . it is therefore appropriate to note the reading promptly after touching the probe to the sample . whereas the probe of the invention is arranged to require no substantial preliminary steps of discharging electrolyte and the like , such quick readings are facilitated . the end of the probe can be cleaned when necessary ( e . g ., when testing gold after brass ) by touching the tip to an absorbent material such as a paper towel or the like , and thereby to wick fresh electrolyte toward the surface . it is advantageous when testing gold , for example , to touch the probe tip to the sample briefly two or three times , until the reading obtained is consistent ( due to sufficient wetting of the probe tip . a brief touch is preferred due to potential decay of the electrolyte over time . for testing relatively pure platinum , a longer touch can be made without adversely affecting the reading obtained . after a sample has been tested , capillary action quickly redistributes the electrolyte in the tip of the probe , enabling a further test after a short time period . this time period for recovery of the electrolyte is minimum where the probe is only briefly touched against the sample to obtain a reading , then withdrawn , because testing in this manner uses the least electrolyte in the galvanic reaction . the probe of the invention is advantageously used to verify quickly the suspected karat values of samples , to screen samples for further testing and the like . in testing items of jewelry , it is possible using the probe to test for purity at different areas of the sample , such as areas adjacent a setting or at a solder joint , or the chain as well as the setting of a pendant . there is minimal damage to the sample , which typically can be cleaned with the mildest of abrasives ( e . g ., a pencil eraser ) to remove all evidence of the test . while a preferred embodiment of the present invention has been described and illustrated in the drawings , such description and illustration are by way of example , and it is to be understood that modifications could be made without departing from the scope or spirit of this invention .