Abstract: A monitoring apparatus which has a plurality of encoders for installation at respective outlying stations, a decoder for installation at a central station and a portable memory module to be carried by a person whose visits to the outlying stations are to be monitored. Each encoder includes a particular configuration of permanent magnets which provides it with its own predetermined magnetic code for identifying the station at which it is installed. This code, in the case of each encoder, is read out and converted to a corresponding binary electric pulse code with the aid of reed switches in the portable memory module, the switches being operated by the fields of respective ones of the encoder magnets when the module is interfaced with the encoder, the electric pulses being inputted to a shift register which is also in the module and which electronically stores the pulse code.