GB2023902A - Testing coins - Google Patents

Abstract

In order to render counterfeiting of coins uneconomic, the invention proposes the manufacture of coin testers with different characteristics between individual coin testers, so that a false coin which happens to be acceptable to one individual coin tester will nevertheless be rejected by another coin tester. It is preferred to vary the characteristics of individual coin testers by varying the ratio between an input capacitance and an output capacitance of a push-pull oscillator. Genuine coins should produce the same output from all the coin testers, whilst false coins produce different outputs from the individual coin testers.

Description

SPECIFICATION Method of reducing the risk of false money in the operation of coin-actuated automatic machines and coin tester for performing the method.

The invention relates to a novel method of reducing the risk of false money in the operation of coinactuated automatic machines and describes coin testers which are particularly well suited for performing the method.

Coin testers are inserted in automatic machines for limiting the risk of false money. Heretofore the procedure was exclusively that coin testers of a certain type were adjusted in an optimum manner to the respective case of use (e.g. "testing of DM 1.--") and were produced equal amongst themselves with the optimum construction found and were used in automatic machines. Coin testers of this kind operate mechanically or electrically or electronically.

Their common characterising feature is that independently of a respective manner of construction used they make mutually similar qualitative statements in respect of the coins to be accepted (= acceptance security) as well as the coins to be rejected (= false money security).

There exists only one optimum adjustment for the respective case of use. This is characterised in that the measurable influences which are produced by a genuine coin - or a group of genuine coins - differ most clearly from the influences which are produced by the coins to be rejected.

In the simplest mechanical coin testers which examine only the diameter, no modifications are possible by adjustment of a different qualitative statement. Admittedly, the possibility of a different qualitative statement capability is given in the numerous constructions of electronic coin testers which have become known, but in comparison with the optimum constructions always means a deterioration.

Consequently, the efforts to limit further the risk of false money is directed heretofore to improving the recognition accuracy. That is to say endeavours are made to increase the discrimination ability of the measuring circuits used in such coin testers and beyond a realisable limit determined by the respective method to increase the quality once more, in that two or more measurements of a different statement in respect of one and the same coin are linked together. Apart from the fact that the physically caused limits referred to below are set to this procedure, the production of such coin testers becomes increasingly expensive and thus their practical usability is restricted to a large extent.

Therefore the invention is based on the object to obtain a very high false money security by the simplest means and for this purpose a novel method as well as also suitable coin testers for performing the method.

According to the invention, it is proposed to produce the coin testers to be inserted in such a manner that they comprise amongst themselves always the same or approximately the same qualitative statement in respect of the coin to be accepted, if the type is the same (e.g. for the examination of DM 1.--), but a respective different one in relation to the articles to be rejected. In this context, always electronic coin testers are referred to below, although the performance ofthe method may also be envisaged for mechanical coin testers, if the procedure is suitable. The proposed measure according to the invention attains that practically "every coin tester is different" in relation to a certain falsification. This need not be attained in the literal sense.When the degree of risk is lower, it may be sufficient to provide a sufficiently large multiplicity of different adjustments in respect of the false money rejection. The aim is not to render a single falseification impossible at all, but to suppress quasi-professional forgeries on a large scale or to frustrate the passing-on of recognitions in respect of existing forging possibilities.

This is to be explained with reference to a practical example: Even electronic coin testers of advanced construction can be outwitted by the fact that articles having the dimensions of original coins are produced the electrical properties of which are similar to those of genuine coins. This can be achieved easily by a combination of different metals, if the electrical parameters of the original coin are measured carefully. According to the state of the art, not only one defined coin tester can be outwitted with such a specimen, but every coin tester of this type. This has led to the fact that regionally forgeries on a large scale occur which originate from a recognition once obtained in respect of a defrauding possibility. When - as proposed - "every coin tester is different" this is no longer possible.A defrauding possibility once found has then practicalvalue only in. respect of the coin test on which it was tried out. A different coin tester would then need also a different kind of false money. This false money risk can be reduced with a probability which increases with the number of different quality adjustments provided. Since the different adjustments provided in the production obtain a haphazard spatial distribution by way of the sales channels, it is sufficient for obtaining a satisfactory false money security when, taking this condition and the respective degree of risk into account, a satisfactory statistical probability is obtained for the fact that adjacent coin testers are different.

The proposed method does not mean that the coin testers used for its performance must be of better quality. It is sufficient that they are sufficiently different in respect of false money.

It may be found in an individual case that the measures for obtaining the different false money rejection lead to additional costs and render then production more expensive. As may be seen from the following statements, this need not necessarily be the case. In any case, such costs would be lower than the costs which arise from the present procedure. These arise on the one hand owing to the widely spread accummulation of false money in a multitude of automatic machines and additionally from the measures which become necessary after such forgeries have become known: modification or dismantling of a large number of coin testers with the risk not removed - namely that of another type of forgery, likewise again in respect of large number of coin testers.

However when forgeries cannot be used any more in a large number of automatic machines, the production of forgeries becomes uneconomical.

In a figurative sense, the proposal of the invention to render "every coin tester different" means the introduction into coin testing of the principle of the safety lock system.

The largest number of different qualitative statements of the coin testers in respect of false money may be obtained if electronic measuring and testing circuits are used. Afirst measure according to the invention which is realisable with some of the electronic coin testers which have become known, is to operate the coin testers of the respective kind with different frequencies. However, these possibilities are physically very limited. An optimum statement of the genuine coin in relation to rival metal discs is attainable as a rule only within a frequency range of a few 10 kHz, within which admittedly any frequency is selectable, but the qualitative statement hardly changes.

Since a certain coin is always in competition with a multitude of coins of the same material and discs of different materials, there result in practice only two or three frequency bands which are utilisable.

Therefore a further advantageous measure according to the invention is to arrange the measuring probes of the test circuits used at different spacings from the articles to be tested and to attain in this way that the influences produced by a certain article are always different in the individual coin testers.

For simple cases of use a sufficiently large multitude of different adjustments is already attainable by the two measures - different measuring frequencies and different probe arrangements-.

However, just these measures require manufacturing expenditure. The invention therefore proposes a further measure which can be handled better from the manufacturing point of view and which consists in rendering different in respect of their qualitiative statement the test oscillators of the measuring circuits used in the individual coin testers by means of suitable different circuit dimensions. The multiplicity attainable in this way can be increased further in that the proposals referred to above are used additionally.

Proposals how a test oscillator can be modified in the sense of the invention by means of suitable circuitry have not been made yet. Therefore, with reference to a novel test oscillator, the invention describes how this can be attained. The following constructional example is only one of many solutions which may be envisaged and which may be found according to the state of the art with the use of the inventive ideas disclosed.

The novel test oscillator described below is not characterised only by the fact that it can be constructed with a very large multitude of different qualitative adjustments. The disclosures and proposals of the invention made in respect of it are of importance in the same manner when it is not used for performing this method, because it possesses a very high selectivity, its production is very inexpensive, and it can be produced very constant. Moreover, for the first time it offers the possibility to obtain two or even four different measurement statements from only one oscillator and thus constitutes a considerable advance in relation to the state of the art.

As previously stated, when an electrical measuring and evaluating circuit is used which converts to an electrical measuring quantity the change of a parameter of the oscillator oscillations, which change is caused by the movement of a coin through the alternating current magnetic field of the coil of an oscillator, and evaluates this measuring quantity by means of two different threshold circuits providing a tolerance range, as well as controls a sorting device dependently upon the result of this evaluation, a coin tester for performing the method may be characterised in that when the coin testers have the same type, the test oscillators are always operated with different frequencies or that the measuring probes are always arranged with different spacings from the coins to be measured.Or even that the oscillator circuits are differently influenced as to their statement ability by means of different circuit dimensioning, such as shown by the example described below of an advanced embodiment.

The greatest multiplicity of possible adjustments is obtained when, according to a fundamental proposal of the invention, the test oscillator is constructed in the form of an LC-push-puil oscillator with two complementary amplifiers, its feed-back resistance is constructed as an inductance and this inductance is subjected to the influence of the coin as a measuring inductance. it is attained thereby that on the one hand the inductance operates like a resistance which is variable by the coin, and thus can influence the feedback of the oscillator, and on the other hand the measuring inductance, being one of the members determining the frequency, can change the frequency of the oscillator in the attenuated state, likewise dependently upon the influence of the coin.

Thus it is attained that different qualitative adjustments can be attained even when the measuring frequency applying to all coin testers is the same and the construction of the measuring probes is the same, if according to a further proposal of the invention this frequency is adjusted with differently large inductances. As explained, in the embodimenf according to the invention the coin influences directly by means of the measuring inductance the feedback of the tuned circuit and the frequency.

Although the percentage of the influence of a certain coin is always the same, nevertheless the absolute influence changes dependently upon the inductance selected; the influence is the greater, the greater the inductance is. Thus, by always selecting different L:C ratios at a certain frequency, always different influences of feedback and frequency can be obtained; as practical investigations have shown - this permits at least six qualitatively clearly different adjustments already at a certain frequency. This means that by dimensioning different L:C ratios for a push-pull oscillator, the latter permits a six times larger number of possible adjustments compared with oscillators used hereto for coin testing.

It is a further very advantageous proposal of the invention to arrange the oscillator circuit in a capacitatively asymmetrical manner, i.e. to arrange capacities of different magnitudes at the input and at the output. The input capacities may be selected larger than the output capacities, and vice versa. And the ratio of the partial capacities relatively to each other may be varied. In each case it is obtained that the tuned circuit is detuned. It has been observed that not only different distortions of the sinusoidal oscillations occur at the input and at the output dependently upon the kind and magnitude of the asymmetry - but also that very clear differences of the qualitative statement adjust themselves.

These depend upon the side on which the large capacitance is arranged: on the side to which the larger capacity is connected, the sinusoidal halfwaves are distorted to a variable from only little distortion to as much as to give them almost an approximate rectangular shape. The evaluation of the changes of these half-waves results in a very good statement on the kind of material of the examined coin for a less well pronounced sharpness of the separation of coins of the same material.

The side to which the smaller capacity is connected shows a nearly sinusoidal shape of the half-waves and the evaluation of the changes of these half-waves is very well suited for the differentiation of coins of the same material.

Also, it is not unimportant to which side the larger capacity is connected. If the larger capacity is connected to the output side, a weaker frequency shift results than in the opposite case - referred to examined coins of the same size but different material. Dependently upon the evaluated parameters of the oscillator oscillation, this feature, too, may be utilised in a manner designed forthe respective case of use, when selecting the adjustments to be made in the oscillator.

Rlwever, the most important change in the qualitative statement of the test oscillator results from a change in the asymmetry in the capacitative connection. Ratios of the partial capacities of from 1:1.05 to 1:20 and 1.o5:1 to 20:1 have been investigated in steps of 0.05, and for each step considerably differing quality statements have been detected. Together with the other measures previously described this produces such a multiplicity of adjustments that an effective performance of the method is ensured.

At the same time the adjustments observed are reproducible and do not render the production difficult, in contrast to the measures referred to above (varying frequencies, varying probe adjustments) which would also be possible in some ofthe coin testers which have become known.

The large multitude observed of possible adjustments may be explained if one imagines - theoretically - that the asymmetrically constructed push-pull oscillator consist of two parallel tuned circuits each of which has a different resonant frequency. The push-pull oscillator adjusts itself automatically to that frequency at which the loop amplification has a maximum magnitude. It operates on the slope of the oscillation curve of both (imaginary) parallel tuned circuits, namely at the intersection of the two (imaginary) curves. This condition explains also the distortion of the half-waves and the respective different statement in respect of certain materials dependently upon the arrangement selected. The attainable multitude of the adjustments may be increased once more by suitable circuit measures at certain points of the proposed oscillator, as will be stated below.

One of the measures serving for this purpose is the aimed attenuation of the oscillator tuned circuit which, dependently upon the pre-damping performed, has an effect to a different extent upon the qualitative statement ability of the oscillator. For this purpose, for example a damping winding may be arranged adjacent the measuring inductance, or in coin testers of the same type a damping winding with a respective different influence.

According to a further proposal of the invention, the amplifier of the oscillator may be connected through a resistance to the positive operating voltage and obtain in this manner an attenuation which again can be varied in many ways dependently upon the selected resistance.

It has been found to be particularly advantageous to connect the amplifier of the oscillator to earth by way of a resistor and to dimension the latter differently in the individual coin testers. Such a resistor to earth exerts a considerably stronger effect upon the attenuation and thus the qualitative state ment of the oscillator than a resistor to the operating voltage, since the latter resistance must always be considered in conjunction with the internal resistance of the voltage source. However by means of a resistance to earth a particularly sensitive and strongly marked influence upon the qualitative statement ability is obtainable in many steps.

Finally, it can be attained in a known manner than the tuned oscillator circuit is operated with an amplification which is dependent upon the operating voltage supplied. Thus by means of different operating voltages always a different amplification may be adjusted and by means of this measure the state ment ability may again be influenced.

The measure described relate so far to a measuring and testing circuit which evaluated only a single parameter change and attains the performance of the method by way of a very great difference between the individual apparatus. This means an economic manufacture which may go hand in hand with a very good selectivity, e.g. when the proposed push-pull oscillator is used.

In special cases, it may be necessary to provide two independent measuring statements in respect of one coin and to interlinkthem in a known manner and to utilise them for the acceptance of this coin.

Therefore it is a further object of the invention to make available a plurality of qualitatively different statements in the most economic manner which then may be utilised all together or in any desired combination. This is effected in accordance with the proposal of the invention in that the push-pull oscillator previously described is employed which offers at the input end a'nd at the output end changes of a parameter of the oscillator oscillations which each result in different statements in respect to the same coin. This proposal of the invention has the economically importance advantage that only one single test oscillator need be used with the resulting advantages in respect of savings of cost and space.

For performing the method and for attaining a still greater multiplicity of differences between individual coin testers, it is then possible to take the evaluation quantity alternately off the input or the output of the oscillator, and in the special cases referred to signals may be taken off both ends of the oscillator also and evaluated in combination.

The following examples describe the invention further and contain further elaborations of the invention: In the drawings: Figure 1 illustrates a constructional example for an LC-push-pull oscillator in accordance with the proposals ofthe invention; Figure 2 illustrates the qualitative statements of known electronic coin testers in relation to a genuine coin and to four falsifications; Figure 3 illustrates the qualitative statements of coin testers according to the invention in relation to the same examined coins; and Figure 4 describes an example in which altogether four different qualitatively differing measuring state ments are obtained by the proposed test oscillator.

Figure 1 illustrates a npn-transistor 1 and an npn-transistor 2 the two bases of which are con nected by means of a lead 4. A measuring inductance 5 is connected between the leads 3 and 4. The emitter of the transistor 1 is connected to a positive operating voltage - under certain circumstances by way of a resistor 10; the emitter of the transistor 2 is connected to earth - under certain circumstances by way of a resistor 20. The resonant circuit capacity is formed by capacitors 21 and 22 connected one between earth and the input of the resonant circuit (21) and one between earth and the output (22).

Numerous adjustment possibilities already described in respect of the qualitative statement may then be performed.

For example, the two partial capacities 21 and 22 may be selected to have equal magnitude and be connected capacitatively to the resonant circuit in a symmetrical manner. In this case the same curve shapes of the oscillator oscillations and also similar changes are obtained under the influence of a coin at the point 210 as well as at point 220. The changes of certain parameters of the oscillator oscillations effected by certain examined coins, however, may be made different in a purposeful manner depen dently upon the respective ratio selected for the measuring inductance 5 and the capacity 21 and 22 of the resonant circuit, as already explained above.

At each of these possible adjustments the state ment ability of the test oscillator changes on the one hand dependently upon a resistance 10 which may be selected from numerous values, and onthe other hand to a particular degree dependently upon a resistance 20 which is likewise selectable from numerous values.

As likewise previously described, one of the capacities (21,22) may be selected larger than the other one. A doubling of the adjustment possibilites results further from the fact that the larger capacity may be connected to the input as well as to the output of the amplifier. Since in both cases different changes, above all - but not exclusively - of the frequency shift result under the influence of the examined coins, this means also an increase of the number of adjustable different qualitative statements of the test oscillator.

The complex processes within the resonant circuit cannot be calculated as to their multitude even with the assumption of static conditions and certainly not with the observation of the dynamic progress during an influence by a coin.

However, the value of the circuit iilustrated is not reduced for the purposes of coin testing thereby. It is sufficient for the practical application that the individual circuit variants are reproducible in respect of their qualitative statement; this has been detected with regard to a large number of adjustments. The adjustment of a coin test circuit and thus the test oscillator is effected always in view of a certain genuine coin or a group of genuine coins. In relation to these an evaluable change of a certain parameter of the oscillator oscillation must be determinable.

Beyond this it is sufficient for the performance of the method to known with certainty that every other adjustment effected also produces other qualitative statements in respect of falsifications of other materials.

In contrast to the methods practised heretofore it is not the one optimum adjustment which is to lead to the success, but the multitude of different adjustments. The example shown proves that the multitude required for performing the method is also practically possible and that it need not be bought for a high technical expenditure.

In Figure 2, the voltage signals of five different examined coins is always entered along the ordinate axis of the image; this amount is produced by them in different samples I - V of this type of coin tester.

The values of the individual examined coins are connected together by the characteristic lines 1 - 5.

These characteristic lines have an approximately parallel course which expresses the identity or constancy of the test circuits produced (not in accordance with the invention) and amongst themselves and in relation to the characteristic line 1 for a genuine coin (DM 1.--) they have a spacing by which the selectivity or quality is expressed. The characteristic line 2 extends approximately identical with the characteristic line 1 and expresses the measuring results of a forgery executed by a combination of different metals; this forgery is accepted by all coin testers shown, because it lies within a toierance field to be imagined to lie about the characteristic line 1 and to be caused by the stray values of genuine coins.

The same examined coins measured at the same measuring frequency and with the same construction and arrangement of the measuring probe in respect of the spacing of the latter from the examined coins are again used in the image illustrated in Figure 3, this time in respect of their measuring results by coin testing circuits according to the proposals of the invention, namelyforfive different haphazardly selected adjustments from a number extending towards infinity of possible adjustments.

1 denotes again the characteristic line of the genuine coin (DM 1.--), 2 denotes the false money disc of the first example, 3 denotes a copper disc, 4 denotes a brass disc, and 5 denotes an iron disc.

I' denotes the adjustment of the test oscillator which uses a symmetrical capacitative arrangement and the same frequency together with an inductance which is smaller than in the adjustment denoted by II'.

III' denotes an adjustment of the test oscillator which comprises the same L:C ratio for the resonant circuit and the same inductance as in example 1', but at the input end a larger capacity, namely in the ratio 1.05:1. IV' denotes the same dimensioning, but with a ratio of 2.5:1 of the partial capacities. V' denotes an adjustment of the test oscillator having the adjustment of example IV', but an additional resistance of 70 ohm to earth.

The characteristic lines 1' to 5' have a non-parallel course by which is expressed the different qualitative statement in respect of false money. The adjustment I' provides nearly the same results as the coin tester described in Figure 2: there, too, it is possible to imitate the result from a genuine coin by means of the respective forgery. However the measuring statements ofthe individual examined coins in relation to the genuine coin is different for all five constructions I' to V', both in respect of the amount as well as the tendency.The image expressyes that minor changes of the dimensioning of the test oscillator which may easily be performed during the production and do not increase the cost thereof, manifest themselves in a fundamentally different statement of the individual coin testers in respect of the coins to be rejected.

Figure 4 illustrates by way of example how a coin tester having a four-fold quality statement in respect of each coin to be tested may be constructed with the use of the idea of the invention.

1 illustrates symbolically the test oscillator according to Figure 1, its measuring inductance being denoted by 5. 210 denotes the tap at the input end for an HF alternating voltage, 220 denotes the tap at the output end. The one measuring voltage taken off is fed to the rectifiers 221 and 2110, the other one is fed to the rectifiers 221 and 2210. According to a proposal of the invention, the rectifier circuits 211 and 221 may be designed for the rectification of the lower half-waves of the alternating voltages, the rectifiers 2110 and 2210 may be designed for the rectification of the upper half-waves.As stated already above, the alternating voltages prevailing at the points 210 and 220 have different frequencies, curve shapes and amplitudes and change differently under the effect of a coin, so that fundamentally two qualitatively different statements are produced. The same difference both in the kind of half-waves as also the change under the influence of a coin, however, is always produced for the lower and the upper half-waves. The test oscillator according to the invention which operates without centre reference between operating voltage and ground, finds its own centre dependently upon the asymmetry of arrangement selected. The curve shape as well as the amplitude of the upper and lower half-waves are different for this reason.

Thus in the proposed novel test oscillator altogether four different measuring statements are available which either may likewise be employed in alternating mixture for obtaining a highest possible number of different coin testers, or which - as described herein merely by way of example - permit a coin tester for three types of coin and four-fold measurment of each coin to be contructed in the most economical manner which may be envisaged.

The evaluation of these four measurement statements may be effected in a known manner. Evaluation circuits of numerous constructions have beome known. In the example shown the rectified measuring voltage is supplied by the rectifier 211 to an amplifier 212 and then offered to three window circuits (213,214,215) each of which is adjusted to a certain coin range. For example the window circuits 2140,214,224 and 2240 serve for assessing the measurement statements for a coin at 10 p. Their outputs are combined in a four-fold AND gate 232, so that the latter starts a timing circuit 235 only if four positive statements in respect of the measured coin are present, the timing circuit in turn being capable of delivering a signal by way of the output 238 to apparatus connected thereto as well as being capable of energising by way of the OR gate 240 a driver stage 241 for a magnet of a gating switch control.

Claims (25)

1. Method of reducing the risk of acceptance of false money in the operation of coin actuated automatic machines, characterised in that coin testers are produced and/or are used in respective automatic machines, which in relation to the coins to be accepted, comprise amongst themselves the same or substantially the same qualitative statement ability (= acceptance security) and, in relation to articles to be rejected, comprise amongst themselves a different qualitative statement ability (= false money rejection).

2. Method according to claim 1, characterised in that the number of different adjustments used of the false money rejection is so great that taking into account the haphazard distribution of the coin testers to the individual erecting locations of the automatic machines, such distribution resulting from the manufacturing process and the sales paths, a satisfactory statistical probability is produced that coin testers in adjacent automatic machines are not equal in respect of their false money rejection.

3. Method according to claim 1 or 2, characterised in that for the purpose of obtaining the different adjustment of the false money rejection, coin testers with an electronic measuring and evaluating circuit are used and these coin testers are adjusted differently in a multiplicity which is attuned to the respective degree of risk.

4. Method according to claim 3, characterised in that the test oscillators of the measuring and evaluating circuits provided for the acceptance of a defined type of coin (or group of coin types) are operated with different frequencies in the individual coin testers of a type.

5. Method according to claim 3 or 4, characterised in that measuring probes of the measuring and evaluating circuits provided for the acceptance of a defined type of coin (or group of coin types) are arranged with different spacings from the coins to be measured in the individual coin testers of a type.

6. Method according to claim 3,4 or 5, characte- rised in that test oscillators of measuring and evaluating circuits provided for the acceptance of a defined coin type (or group of coin types) are constructed differently in the individual coin testers of a given type in respect of their qualitative statement ability by means of different dimensioning of the circuit.

7. Coin tester for performing the method according to one or more of claims 1 - 6, comprising an electrical measuring and evaluating circuit which in use converts to an electrical measuring quantity a change of parameters of an oscillator oscillations, which change is caused by the influence of a coin upon the alternating current magnetic field of a coil of an oscillator, and assesses these measuring quantities by means of threshold circuits, as well as controls a sorting device dependently upon the result of this evaluation, characterised in that the test oscillators of coin testers of the same type are operated with different frequencies.

8. Coin tester for performing the method according to one or more of claims 1 - 6, using an electrical measuring and evaluating circuit which in use converts to an electrical measuring quantity a change of parameters of an oscilator's oscillations, such change being caused by the influence of a coin upon the alternating current magnetic field of a coil of an oscillator, and evaluates this measuring quantity by means of threshold circuits as well as controls a sorting device dependently upon the result of this evaluation, characterised in that the coils of the test oscillators of coin testers of the same type are arranged with different spacings from the coins to be tested.

9. Coin tester for performing the method according to one or more of claims 1 - 6, using an electrical measuring and evaluating circuit which in use converts to an electrical measuring quantity a change of parameters of an oscillator's oscillations, such change being caused by the influence of a coin upon the alternating current magnetic field of a coil of an oscillator, and evaluates these measuring quantities by means of threshold circuits as well as controls a sorting device dependently upon the result of this evaluation, characterised in that the oscillator circuits of the measuring and evaluating circuits of coin testers of the same type.are differently influenced as to their statement ability in respect of competing articles, by means of different dimensioning of the circuit.

10. Coin tester according to claim 9, characterised in that the test oscillator of the measuring and evaluating ciruit is an LC push-pull oscillator with two complementary amplifiers and that a feedback impedance thereof is constructed in the form of a measuring inductance which is subjected to the influence of the coins.

11. Coin tester according to claim 10, characterised in that a certain measuring frequency of the oscillator circuit of coin testers of the same type is constructed with different ratios of the measuring inductance and the total capacity of the resonant circuit.

12. Coin tester according to claim 9, 10 or 11, characterised in that the oscillator circuit is arranged in a capacitatively asymmetric manner, that is to say comprises differently large cpacities at the input end and at the output end, or that the ratio of the partial capacity at the input end and the partial capacity at the output end is different in coin testers of the same type.

13. Coin tester according to one of the claims 9 12, characterised in that the oscillator circuit comprises a pre-damping means and preferably a different means in coin testers of the same type.

14. Coin tester according to claim 13, characterised in that a damping winding is arranged adjacent the measuring inductance and preferably such a one of different influence in coin testers of the same type.

15. Coin tester according to one or more of claims 9 - 14, characterised in that an amplifier of the oscillator circuit is connected to earth by way of a resistance and preferably by way of differently large resistances in coin testers of the same type.

16. Coin tester according to one of the claims 9 15, characterised in that an amplifier of the oscillator circuit is connected to the positive operating voltage by way of a resistance and preferably by way of differently large resistances in coin testers of the same type.

17. Coin tester according to one or more of claims 9 - 16, characterised in that the oscillator circuit is operated with a certain amplification which depends upon its operating voltage and preferably with different amplifications in coin testers of the same type.

18. Coin tester according to one or more of claims 10 - 17, characterised in that in coin testers of the same type the measuring quantity to be evaluated is tapped off the amplifier partly at the input end, partly at the output end and partly at both ends thereof.

19. Coin tester according to claim 18, characte- - rised in that the lower or the upper half-wave of an alternating HF voltage tapped off is rectified and separately evaluated.

20. Coin tester using an electrical measuring and evaluating circuit which converts to electrical measuring quantities the change of parameters of the oscillator oscillations, which change is caused by the influence of a coin upon the alternating current magnetic field of the coil of an oscillator, and evaluates this measuring quantity by means of threshold circuits as well as controls a sorting device dependently upon the result of this evaluation, characterised in that the test oscillator of the measuring and evaluating circuit is an LC push-pull oscillator with two complementary amplifiers and that the feedback impedance thereof is constructed as an inductance which is subjected to the influence of the coins.

21. Coin tester according to claim 20, characterised in that the LC push-pull oscillator is arranged in an asymmetric manner.

22. Coin tester according to claim 21, characterised in that the oscillator comprises capacities of different magnitude at the input end and at the output end.

23. Coin tester according to claim 21 or 22, characterised in that an HF measuring voltage is tapped off the input of the oscillator and is fed to an evaluating circuit.

24. Coin tester according to claim 21,22 or 23, characterised in that an HF measuring voltage is tapped off the output of the oscillator and is fed to an evaluating circuit.

25. Coin tester according to one or more of claims 20 - 24, characterised in that the upper or the lower half-wave or both, of an HF measuring voltage tapped off, are separately rectified and fed to an evaluating circuit.