Apparatus for detecting electrically conductive coatings on documents

Apparatus for determining the genuineness of a document of electrically nonconductive material coded by means of an invisible electrically conductive surface layer of predetermined conductance in a limited locality thereof, comprising:

• a. a support member for receiving the document in a stationary test position with said surface layer facing the support member;
• b. an array of spaced electrical contact elements fixed on the support member and adapted to engage said surface layer when the document is in said position;
• c. means for engaging the opposite surface of the document and forcing said layer into engagement with the contact elements;
• d. means connecting one terminal of an electrical circuit to at least one of the contact elements engaging said limited locality;
• e. means connecting a second terminal of said circuit to at least one other of the contact elements engaging said limited locality;
• and
• f. indicating means in said circuit responsive to the conductance between said terminals and operable when said conductance is greater than a predetermined value to produce a positive indication that the document is genuine.

Apparatus for determining the genuineness of a document of electrically non-conductive material coded by means of an invisible electrically conductive surface layer of predetermined conductance in a limited area having a location relative to the edges of the document which is defined by the code, comprising:

• a. conductance sensing means including an array of electrically conductive elements, each substantially smaller than said area, said elements being separated by insulating means, said array extending over an area substantially greater than the limited area;
• b. means for engaging the edges of the document to hold said area in cooperative relationship with only a portion of the elements, said portion being defined by the code;
• c. electrical circuit means including only said portion of said elements and responsive to the conductance of said layer;
• and
• d. indicating means operated by said circuit means and effective only when the conductance of said layer is within a predetermined range of said predetermined conductance to produce a positive indication that the document is genuine.