GB2211976A - A thread detecting apparatus for bank notes - Google Patents

Abstract

The thread detecting apparatus comprises a Piezo electric high voltage generator (1) actuated by a hammer (2) supplying up to 6,000 volts. The generator is earthed at (3) and electrically connected to a conductive brush (4). A second conductive brush (5) picks up the current supplied by the generator via a metallised thread (6) embedded or woven in a bank note (7). The brush (5) is connected to a gas discharge tube (8) which registers a measurement of the conductivity of the thread (6) on a standard photocell (9). The gas discharge tube is earthed at (10) to complete the circuit. The conductive brushes (4) and (5) may be replaced by two capacitor plates placed one at each side of the bank note or by one or two rollers. The pair of plates or pair of rollers may further be replaced by a single plate or a single roller having an interwoven capacitive formation. <IMAGE>

Description

A THREAD DETECTING APPARATUS FOR BANK NOTES This invention relates to a thread detecting apparatus for bank notes and more particularly to the detection of metallised threads or metal strips embedded in the bank note paper.

All British and certain foreign bank notes have a metallised thread embedded in the bank note paper. The authenticity of these bank notes can be verified by checking the presence of the metallised thread together with other features of the bank notes.

At present, the verification of the presence of the thread is purely an optical method, ie., the bank note is held up to a light source making the thread visible to the human eye. Modern printing technology has made this feature easy to counterfeit. The present invention proposes to verify the presence of a metallised thread or metal strip by checking its conductivity.

Three main problems exist in testing the conductivity of the thread, firstly to make an electrical contact with the metallised thread without causing damage to the bank note, secondly the metallised thread becomes degraded with use and thirdly the metallised thread can exhibit extremely low conductivity.

An aim of the present invention is to provide a thread detecting apparatus for bank notes which overcomes these problems.

According to the present invention, there is provided a thread detecting apparatus for bank notes comprising a high voltage generator, electrical connecting means to connect the high voltage supply to a metallised thread or metal strip of a bank note and electrical connecting means to pick up the current which is passed along the metallised thread or metal strip and voltage registering means to record the conductivity of the metallised thread or metal strip.

Preferably, the high voltage generator includes a Piezo electrical device.

Conveniently, the electrical connecting means to supply current to and pick up the current from the metalised thread or metal strip are conductive brushes which need not contact the bank note.

In the at present preferred construction the voltage registering means comprises a gas discharge lamp connected to the conductive brushes, the resultant electrical discharge being picked up by a conventional photocell.

Embodiments of the thread detecting apparatus, will now be described, by way of example only, with reference to the accompanying drawings in which; Figure 1 is a diagrammatic view of a first embodiment of a thread detecting apparatus according to the invention; Figure 2 is a diagrammatic view of a second embodiment of the invention; Figure 3 is a diagrammatic view of a third embodiment of the invention; and Figure 4 is a diagrammatic view of. a forth embodiment of the invention.

The thread detecting apparatus comprises a Piezo electric high voltage generator 1 which can be actuated by a hammer .2 to supply up to 6,000 volts. The generator 1 is earthed at 3 and electrically connected to a conductive brush 4. A second conductive brush 5 picks up the current supplied by the generator via a metallised thread 6 embedded or woven in a bank note 7.

The brush 5 is connected to a gas discharged tube 8 which picks up and registers a measurement of the conductivity of the thread 6 on a standard photocell 9 using conventional electronic circuitry. The gas discharged tube is earthed at 10 to complete the circuit.

Many of the worlds bank notes including all the British bank notes are provided with a metallised thread or metal strip embedded in the bank note paper or more recently woven across the note to provide a series of exposes portions of the thread, to enable the bank note to be identified as genuine currency. This feature along with other features such as magnetic ink and watermarks are used to verify the bank note as being genuine. However, the use of an electric current to determine the presence of the metalised thread has presented the problem of having to make electrical contact with the strip without causing damage to the bank note. Another problem is to be able to pass an electric current along the thread if it has degraded and has become broken due to folding, tearing or other damage to the bank note during the life of the currency.A still further problem is the metallised thread may exhibit low conductivity.

The apparatus of the present invention provides a simple and efficient solution to these inherent problems. The voltage generator 1 must generate a voltage high enough to pass a current through the skin of the bank note paper to the metallised thread or metal strip 6 and to pick up the current at a remote point, from where it enters and contacts the thread, to register that a metallised thread or metal strip is present in the bank note and exhibits the conductivity of a genuine bank note thread or strip The selection of the appropriate high voltage range is important because the higher the voltage the greater the sum of the air gaps the current can jump. Conversely, the lower the voltage the lower the sum of air gaps the current can jump and closer contact with the bank note is thus required.It is beleived that in normal circumstances, the metallised thread follows OHMS Law ie., that it is a conductor, but of a high resistivity, once air gaps are introduced the voltage needs to be high enough on ionize the air to permit conduction to occur.

In the prefered construction, a simple high voltage Opto isolator is described but in another application a higher voltage is selected to penetrate the skin of the bank note paper to contact the metallised thread 6 and to jump any breaks in the thread.

In this arrangement the passage of this high voltage current jumping across the breaks in the thread causes an electric discharge which appears as a spark of light at each break in the thread which is clearly viable to the naked eye.

In the embodiment shown in Figure 2 the thread detector comprises a 2MHz power supply 11 which can supply up to 200 volts to a pair of detection plates 12 and 13. The best results are however found to work with a voltage of 5 volts because it is more stable, noise immune and safer. The detection plates 12 and 13 form the plates of a high value capacitor which is in series with a second capacitor 14.

This capacitor 14 is connected to a gas discharge lamp 15 which is sensed by a photocell 16 and is earthed at 17. A bank note 18 having a metal strip 19 is passed between the first capacitor plates 12 and 13 and the gas discharge lamp 15 illuminates if the metal strip is of the correct conductivity.

The product of the two capacitors in series can be calculated as follows: C1xC2 CTOTAL = C1+C2 If the metal strip is not present the CTOTAL value is high but when a strip is present between plates 1 and 2 the CTOTAL value is small allowing current to flow through the gas discharge lamp 15 thus allowing the presence to be detected by the photocell.

The third embodiment shown in Figure 3 is similar to that shown in Figure 2 where like parts have the same reference numerals. The embodiments shown in figure 2 has the plate 12 above the bank note 18 and the plate 13 below whereas in the embodiment of Figure 3 the plates 12 and 13 are either both above or below the bank note 18. However, the two plates are interwoven to give a capacitive formation.

The- fourth embodiment shown in Figure 4 is a further modification of the thread detector shown in Figures 2 and 3 and like parts have the same reference numerals. In this fourth embodiment, the plates 12 and 13 are replaced by rollers 20 and 21 respectively. Once again the rollers are either both above or below the bank note 18 as in Figure 3.

In a modification of the capacitor rollers 20 and 21 these may be replaced by a single roller formed with two seperate interwoven seperate helical elements similar to the interwoven arrangement of the embodiment of Figure 3.

In a preferred arrangement (not shown) the 2MHz power supply has two tappings, one to supply 5 volts and one to supply 170 volts. The 170 volt supply can be used to drive a quartz tube filled with mercury vapour. This arrangement produces a short wave or long wave ultraviolet light which is used to check the authenticity of the paper from which the bank note is made.

Although the roller type capacitor have the advantage that they are in direct contact with the bank note, the reading from the capacitors is more critical than the emobiments of 1 to 3 in that the thread only contacts a tangentical line on the circumference of the roller or rollers, whereas the bank note of the embodiments using conductor plates may be skewed relative to the longitudinal axes of the the plates and provide an accurate reading.

It is understood that various modifications can be made to the thread detecting apparatus of the present invention, for example, the apparatus can be made in the form of a small hand held unit with viable conductivity indicating means which can be brushed across a bank note to give the user an immediate indication of the presence and conductivity of the metallised thread or strip in a bank note to assist in tha authentication of the note as being genuine.

Claims (13)

1. A thread detecting apparatus for bank notes comprising a high voltage generator, electrical connecting means to connect the high voltage supply to a metallised thread or metal strip of a bank note and electrical connecting means to pick up the current which is passed along the metalised thread or metal strip and voltage registering means to record the conductivity of the metallised thread or strip.

2. A thread detecting apparatus as claimed in claim 1, wherein the high voltage generator includes a Piezo electrical device.

3. A thread detecting apparatus as claimed in clain 1 or 2 wherein the electrical connecting means to supply current to and pick up current from the metallised thread or metal strip are conductive brushes which need not contact the bank note.

4. A thread detecting apparatus as claimed in any preceding claim, wherein the voltage registering means comprises a gas discharge lamp connected to the conductive brushes, the resultant electrical discharge being picked up by a conventional photocell.

5. A thread detecting apparatus as claimed in claim 1, wherein the high voltage generator produces a 2 MHz power supply to a pair of plates forming a capacitor, one of the the plates being connected to the voltage registering means via a second capacitor.

6. A thread detecting apparatus as claimed in claimed in claim 5, wherein the voltage registering means is a gas discharge lamp and a photocell.

7. A thread detecting apparatus as claimed in claim 5 or 6, wherein the electrical connecting means are two capacitor plates.

8. A thread detecting apparatus as claimed in claim 7, wherein the capacitor plates are interwoven to provide the capacitive formation.

9. A thread detecting apparatus as claimed in claim 5 or 6, wherein the electrical connecting means is two rollers arranged side by side at one side of the bank-note.

10. A thread detecting apparatus as claimed in claim 5 or 6, wherein the electrical connecting means is a single roller with an interwoven capacitive formation on its circumferential surface.

11. A thread detecting apparatus as claimed in any preceding claim, wherein the high voltage generator has a plurality of tappings to supply different voltages.

12. A thread detecting apparatus as claimed in claim 11, wherein the high voltage supply is 2MHz having two tappings, one tapping leading to a 5 volt supply and the other to a 170 volt supply, the 170 volt supply leading to a quartz tube filled with mercury vapour producing a short wave or a long wave ultra violet light.

13. A thread detecting apparatus substantially as hereinbefore described with reference to and as shown in Figure 1, Figure 2, Figure 3 or Figure 4 of the accompanying drawings.