Abstract: The damping of a coil (L1) can be influenced by an object (1) so that the high-frequency voltage at the coil (L1) depends on the distance (a) of the object (1) from the coil (L1). A constant dc current is superimposed on the high-frequency current through the coil, the dc voltage drop at the coil (L1) which corresponds to the dc resistance of the coil (L1), damping the coil (L1), being influenced by the temperature. The high-frequency excitation (5, 6) of the coil (L1) is controlled by the dc voltage drop in order to compensate for the influence of the temperature on the high-frequency voltage so that the high-frequency voltage depends solely on the distance (a). The high-frequency voltage, having a nonlinear correlation to the distance (a), is linearized in a nonlinear member (15) with a semiconductor element with respect to the distance (a). In this connection, the effect of the temperature on the linearization is compensated for by means of a second semiconductor element.

Abstract: The coins (37) come within the coil field of an oscillator (2,6,7). Within a reverse feedback path of the oscillator amplifier (2), several resistances (11 to 18) are arranged, switchable parallel to each other individually or in any desired combination, by switching transistors (21 to 28), each controlled by a program (29). This makes the amplification controllable step by step. The program (29) has a constant-value-storage (49) with a stored standard value for the amplification in which oscillations cease whenever the field coil area is free and a calibrating resistance (35) is connected parallel to the condenser (7) of the oscillator (2,6,7). For each coin type, two pre-determined standard limit values of amplification are stored in the constant-value-storage (49), with the oscillations ceasing between them in the presence of an acceptable coin (37).

Abstract: The coins (8) are thrown into a common coin channel and affect the field of a coil (2; 52). The coil core consists either of several U- or E-cores (54, 55) arranged at right angles to the coin travel direction (5) in direct or spaced-apart adjacency, or of an oblong core (3) U- or E-shaped in cross section which lies at right angles to the coin travel direction. The spacing of the two outermost cores (54, 55) and/or the length of the core (3) is dimensioned to be of such a size that the coins (8) of varying diameter affect the coil field differently. The circuit for determining the effect on the field can comprise, for example, an alternator supplying the coil (2; 52) and a further coil (7; 53) coupled to the coil and connected to a comparison circuit. By means of the coil checker according to the invention it is possible to reliably test and determine even coins having greatly varying diameters.