Detection plate having pin-shaped attachment means

A detection plate comprising an electronic circuit capable of being activated by a suitable interrogation field to generate a pre-determined signal is provided with pin-shaped attachment means (17) arranged so that the unauthorized detachment of the attachment means results in the interruption of an electrical conductor (10) forming part of said electronic circuit. In the embodiment the detection plate is an earmark for animals and the attachment means includes a pin integral with the plate. The pin has a point which can be pushed through an animal's ear and the device can then be fastened on the other side of the ear. The pin includes a conductive loop which extends into the point and is connected to the electronic circuit of the plate.