is detected by detector circuitry 26 which informs the control circuitry 28 likewise the inductance of the first coil 32 of the sensor circuitry 16 2 also has a change in its inductance which is detected by the detector circuitry 36 which in turn informs the control circuitry 38 the control circuitry 38 then controls the drive circuitry 37 to modulate a current in the first coil 32 the modulating current in the first coil 32 induces an equivalently modulating current in the first coil 22 of the sensor circuitry 16 1 the detector circuitry 26 detects the modulation of the induced current in this way the control circuitry 38 of the sensor circuitry 16 2 can transfer data identifying the second radio transceiver device 2 2 to the radio transceiver device 2 1 the detector circuitry 26 obtains the identity of the second radio transceiver device 2 2 and provides it to the control circuitry 28 which in turn provides it to the processor 14 1 the processor 14 1 on receiving the identity of the second radio transceiver device 2 2 controls the low power radio transceiver circuitry 10 1 to transmit packets to the low power radio transceiver circuitry 10 2 of the second radio transceiver device 2 2 using radio packets having a header comprising the identity of the second radio transceiver device 2 2 the control circuitry 28 of the sensor circuitry 16 1 can then via the drive circuitry 27 modulate a current in the first coil 22 and thereby transfer its identity to the second radio transceiver device 2 2 when the second radio transceiver device 2 2 is brought into the predetermined intimate physical relationship with the.