referring now to fig1 - 3 , a testing apparatus for analyzing the concentration or purity of silver within a specimen being tested can best be seen . this testing apparatus 10 incorporates the principles of the instant invention . in general , the testing apparatus 10 is used to analyze the concentration of silver , but is believed to be operable in conjunction with testing the content of gold or possibly other precious metals , such as platinum , and is based on measuring the rate of change of current or voltage shift through the test specimen as a resistive layer is established between the probe and the specimen being tested . an exemplary embodiment of the pen probe 11 forming part of the testing apparatus 10 is shown in fig2 , which is a cross - sectional view of the exemplary hand - held pen probe 11 . the pen probe 11 is preferably formed with a generally cylindrical body 12 , which can be made of plastic or other substantially electrically non - conductive material . a top cap 13 is coupled to an electrical wire 14 fitted with a jack 15 to facilitate a detachable electrical connection of the pen probe 11 to the circuitry 30 within the meter 20 . the end of pen probe 11 , which is placed into contact with the specimen to be tested , is formed as a fiber tip 16 that is in communication with a reservoir 17 containing a supply of electrolyte , such as a saturated solution of ammonium chloride , or other solutions that will be operable to form a resistive layer on the specimen being tested . the fiber tip 16 is preferably provided with a thin platinum wire 18 , which is preferably embedded into the fiber tip 16 and extends into the reservoir 17 . a second thin platinum connecting wire 19 couples the platinum wire 18 to the wire 14 at the top cap 13 of the pen probe 11 . the wires 18 , 19 provide for the passage of electrical current into the electrolyte which is absorbed into the fiber tip 16 and is deposited onto the specimen being tested . the testing apparatus 10 further includes a meter 20 including a housing 21 in which is mounted a printed circuit board 22 including a port 23 to which the jack 15 can be detachably connected . also electrically connected to the circuitry 22 is a test pad 25 that is preferably formed as a copper pour having a surface coating of gold . the meter 20 is also constructed with a light - emitting diode ( led ) indicator bar 24 that reflects the results of the testing of the sample , as will be discussed in greater detail below . in addition , the housing 21 supports a calibration switch 26 that is operable to initiate the calibration procedure , as will also be described in greater detail below . the housing 21 can also support other leds that reflect the status of the operation of the testing apparatus 10 , such as a ready light 27 , a power - on light , a battery status light , etc . the meter 20 can also have a three position on / off switch 29 that is movable to an off position , an external power position , and a battery power position . the housing 21 also supports a port 28 for connection to a source of external power , such as 110 vac electrical current through the use of an adapter ( not shown ). the electronic circuitry 30 is reflected in the schematic diagram of fig3 . either a battery ( not shown ) or a source of external electrical power connected through the port 28 provides an electrical current i net into the circuitry 30 . the current i net reaches the node 32 and is divided into a first current flow i pan and a second current flow i sense . with the silver specimen s placed on the test pan 25 and the probe 11 touching the silver specimen s a circuit is completed for the passage of i pan and i sense to ground 33 . because the initial resistance of the silver specimen s is minimal , the current i pan is substantially greater than the current i sense which must pass through the diode d 1 , diode d 2 and the resister r 25 to reach ground 33 . as the probe 11 touches the silver specimen s , the electrolyte in the probe 11 deposits on the silver specimen s and starts corroding the adjacent surface of the silver specimen s by forming silver oxide between the fiber tip 16 and the silver specimen s . the layer of silver oxide is a resistive layer that impedes the flow of electrical current . thus , as the layer of silver oxide increases , the current i pan decreases and , due to kirchhoff &# 39 ; s law , the current i sense increases . the principle is demonstrated in fig4 and 4a . essentially , the pan 25 , silver specimen s and probe 11 form a variable resistance r 1 due to the growing layer of silver oxide between the probe 11 and the silver specimen s . the portion of the circuitry 30 between the node 2 and the ground 33 , which includes the diode d 1 , the resister r 25 and the diode d 2 , is a fixed value resistance . as the variable resistance r 1 increases with the building of the silver oxide layer , the current i pan decreases , resulting in the current i sense increasing . a prototype of the testing apparatus 10 is depicted in fig5 where the fiber tip 16 of the probe 11 is in contact with the silver specimen s , seated on the pan 25 . the led indicator bar 24 is divided into four sets of leds , although in the commercial embodiment all four sets of lights will not likely be required . in the first set 36 of leds , the lights cycle as the tests , as described in greater detail below are conducted . the first set 36 of leds provide an indication that the testing is being conducted . the second set 37 of leds provides an indication of the charge remaining in the pen probe 11 with the second set 37 of leds providing a visual indication of the status of the pen probe 11 as far as conducting a subsequent test . associated with the second set 37 of lights is the ready light 27 that provides a reinforcement of the status of the pen probe 11 to conduct another test . the third set 38 of leds is the concentration indicator with each respective led indicating a different concentration or purity of silver in the silver specimen s . for example , the led at one end could indicate 99 . 99 % silver content , the next led 95 % silver content , the next led 92 . 5 % silver content ( which is sterling silver ), the next led 90 % and the last led perhaps less than 80 % silver . between the second set 37 and the third set 38 of leds , a red led could be used to provide an indication that no silver content was found during the test . the fourth set 39 of leds could be used to provide a count down of tests before a new calibration of the apparatus 10 is required . referring again to fig1 , the adc 1 and adc 2 sensors provide a reading of the current i sense on opposite sides of the resister r 25 . the test procedure operates to take a predetermined number of readings by the sensors adc 1 and adc 2 which are sent to the microprocessor 40 which calculates the rate of increase in the current i sense . the average rate of increase over the predetermined number of readings being taken is then compared to a calibration sample to derive the silver content within the specimen s being tested . the basic principle in making this derivation is that the higher the silver content in the silver specimen s , the faster the silver oxide layer grows from the deposit of electrolyte from the pen probe 11 . therefore , the faster the rate of increase in the i sense current , the greater the percentage of silver in the specimen s . preferably , the apparatus 10 will take eight readings of the current i sense before the testing of the specimen s is completed . presently , calibration of the apparatus 10 is desired after every four or five tests of specimens s . calibration is conducted by utilizing a known silver content test specimen s , such as a 92 . 5 % sterling silver specimen . the calibration button 26 is pressed to begin the calibration sequence so that the apparatus 10 knows to save the test results in the microprocessor 40 for comparison with the subsequent tests of the unknown silver specimens s . thus , the count down set 39 of leds provides a visual indication as to how many unknown test specimens can be tested before a re - calibration of the apparatus 10 is needed . built into the microprocessor 40 can be a block on further testing of unknown specimens s once the count down has diminished to zero , unless a calibration sequence is conducted . once a test of a silver specimen s has been conducted , the pen probe 11 has to be discharged of residual current , i . e . cooled down , before another test can be conducted . presently , that cool down period is approximately 6 - 8 seconds , which can be programmed also into the microprocessor 40 to prevent any testing unless the cool down period has expired and the ready light 27 is illuminated . thus , the power supply to the pen probe 11 is preferably modulated between on and off by the microprocessor 40 with the power being turned off for the cool down period . in operation , the apparatus 10 is powered on through either an internal battery ( not shown ) or by a connection to a ac power source to provide , preferably , a 3 . 3 volt connection at the power input point 28 . first , the apparatus 10 must be calibrated by depressing the calibration button 26 and placing a known silver content specimen s , such as a sterling silver ( 92 . 5 % silver content ) specimen , on the pan 25 . the fiber tip 16 of the pen probe 11 is then pressed against the known specimen s to deposit electrolyte on the specimen s while the current i pan is passed through the pan 25 , silver specimen s and pen probe 11 . the apparatus 10 takes preferably eight readings of the current i sense to determine a rate of change of the current i sense , which is proportional to the rate of growth of the silver oxide layer forming on the known silver specimen s due to the chemical reaction with the electrolyte . once the eight readings have been completed , the microprocessor 40 stores the average rate of change in current i sense as the calibration sample for future comparisons . once calibrated , the pen probe 11 is passed through a cool down period by the microprocessor 40 turning the power off , before the apparatus 10 is ready to conduct a test of an unknown specimen s . when the ready light 27 is illuminated , the unknown silver content specimen s is placed onto the pan 25 and the fiber tip of the pen probe 11 is placed against the specimen s to deposit electrolyte thereon . as with the calibration procedure , the apparatus 10 takes eight readings of the i sense current and calculates an average rate of increase in the current i sense over the eight readings . because the rate of growth of the silver oxide layer is proportional to the concentration of silver in the unknown test specimen s , the rate of increase in the current i sense will be reflective of the purity of the silver specimen . the rate of increase in the current i sense is then compared to the calibration reading stored in the microprocessor 40 and the purity of the silver is derived . after the test of the unknown silver content specimen s has been completed , the microprocessor 40 again shuts off the power to the pen probe 11 so that the accumulated charge therein can be dissipated and the pen probe 11 cooled down . after the requisite cool down period , the microprocessor 40 illuminates the ready light 27 and another test of an unknown silver content specimen s can be conducted as described above . for each test of an unknown silver content specimen s , the count down led set 39 is manipulated until ultimately reaching zero , whereupon a new calibration of the apparatus 10 is to be conducted , as described above , to enable another series of testing of unknown silver content specimens . one skilled in the art will recognize that this principle of comparing the rate of increase , as opposed to simply measuring a voltage generated , of the growth of a resistive layer being formed on a test specimen can be extended to any precious metal specimen that can grow a resistive layer . thus , while the preferred embodiment of the instant invention is described above with respect to testing for the purity of silver in an unknown silver content specimen , the same principle can be used to test for other precious metals , including gold . it will be understood that changes in the details , materials , steps and arrangements of parts which have been described and illustrated to explain the nature of the invention will occur to and may be made by those skilled in the art upon a reading of this disclosure within the principles and scope of the invention . the foregoing description illustrates the preferred embodiment of the invention ; however , concepts , as based upon the description , may be employed in other embodiments without departing from the scope of the invention .