This invention relates to coin testing apparatus of the kind comprising at least one magnetic inductor for generating or detecting an oscillating magnetic field with which a coin to be tested interacts.

Many kinds of such apparatus are widely available commercially, and normally they use several such inductors for producing (and in some instances receiving) a plurality of magnetic fields which interact with the coins to be tested in different ways. The present invention concerns the structure of such inductors and their positioning within the apparatus, and reference is made to GB-A-1 452 740 and GB-A-2 094 008, for example, for further information as to how other aspects of such apparatus may be arranged and operated.

It is desirable, ideally, that the inductors having a particular function in each of the coin testing apparatuses manufactured to a given design should have identical operating characteristics. This is not achieved in practice due to tolerance variations in the actual construction of the inductors and in their assembly into the apparatuses.

Typically, an inductor comprises a core, frequently annular in shape, of high magnetic

permeability material having a recess in its front face, the recess also being annular in the case of an annular core, and an inductive coil located in the recess. For the purposes of the present specification including the claims, the term "front face" will be used in respect of that face of the core which in operation faces towards a coin being tested, and also in respect of that face of the coil which faces towards the coin. The invention involves positioning the front face of the coil in predetermined relationship to the front face of the core, in assembling the inductor. We have found that this is an important factor in helping to achieve uniformity of operating characteristics as between all inductors made to a given design.

The invention also involves positioning the front face of the coil in predetermined relationship to a structural part of the apparatus adjacent to a passageway along which coins to be tested pass, and hence in predetermined relationship to the passageway and also, ideally, to coins which pass along the passageway.

More specifically, the invention provides coin testing apparatus of the kind comprising at least one magnetic inductor for generating or detecting an oscillating magnetic field with which a coin to be

tested interacts, wherein said inductor comprises a core of high magnetic permeability material having a recess in its front face for receiving an inductive coil, an inductive coil having a front face, first abutment means fixedly associated with the coil, the coil being located within the recess and the first abutment means being arranged to contact the front face of the core to determine the position of the coil front face relative thereto, and second abutment means fixedly associated with the coil, said second abutment means contacting a structural part of the apparatus adjacent to a passageway along which coins to be tested pass, so as to determine the position of the coil front face relative to said structural part and hence relative to the coin passageway.

In order that the invention may be more clearly understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawing, which is a cross- section through the coin passageway of a coin testing apparatus and an associated inductor.

As is common in the art, a coin passageway 2 is defined by side walls 4 and 6 at the bottom of which is a coin track 8 on which coins, such as coin 10, roll in a direction perpendicular to the drawing past one or more inductors such as the inductor 12 shown.

The passageway is inclined to the vertical so as to ensure in so far as possible that the coins always roll past the sensors in contact with wall 4, this being for the purpose of reducing variable factors that would hinder accurate and repeatable testing of the coins.

The inductor 12 comprises an annular core 14 of high magnetic permeability material such as ferrite, having a central hole 16. The front face of the core 14, i.e. the face directed towards coin 10, has an annular recess 18 extending deeply into it, leaving the front face effectively in the form of annular inner and outer front face portions 20 and 22 respectively. An annular coil 24 is wound on a former or bobbin having front and rear radially outwardly projecting flanges 26 and 28 joined by a central cylindrical part 30, and the coil fills the space, in the axial direction, between flanges 26 and 28. In assembling the inductor, the former with the coil 24 on it is inserted into the recess 18 until the margin of the rear surface 32 of flange 26, acting as a first abutment, contacts portion 22 of the front face of the core. Since in this condition the positions of the front face 34 of the coil 24 and of the front face 22, 20 of the core 14 are both being

determined by the surface 32, and surface 32 and the rest of the former (26, 28, 30) can readily be manufactured (e.g. by injection moulding in plastics) to very small tolerances, the relative positioning of coil and core front faces 34 and 20, 22 can be made constant to within very small tolerances throughout a large number of such inductors.

The complete inductor is secured to the rear of a thin part 36 of wall 6 by bringing the front surface 38, which acts as a second abutment, of flange 26 into contact with the rear of wall part 36 with a very thin layer of (e.g. isocyanate) adhesive between them. For protection and more secure fixing, the entire inductor may then be encapsulated by a body of resin (not shown) applied over its rear side.

The wall 6, and particularly its thin portion 36, can be injection moulded from plastics material, as is usual, to very small tolerances in thickness, and so can the flange 26 of the former, as has been mentioned. Consequently, since the distance between the front 34 of the coil and the inner surface of passageway wall 6 is simply the sum of these two thicknesses, that distance can also be made constant to within very small tolerances throughout a large number of apparatuses. That helps to ensure that the variation (between one apparatus and another) in the

relative positions of the coil front face 34 and the nearest face of the coin to be tested is influenced as little as possible by dimensional factors other than the thickness of the coin itself, and this is important for reliable and repeatable results in the testing of coins.