features a tool connector 21 for interconnecting the various tools e g 12 a b and c to the joint part 18 the connector could be a simple screw type fixing fig2 shows in more detail the electrical arrangement illustrated in fig1 in this fig the tools 12 a b and c can be seen housed in rack 40 the tools including probe 12 a have electrical contacts 16 at the releasable joint part 18 that co operate with complimentary contacts 15 on the universal mounting of the joint part 17 two of the contacts 16 are connected to probe data store 19 in this instance a programmable integrated circuit pic the pic has a digital code embedded in memory therein which can be recognised by the processor 20 a similar pic 32 is used as a data store in each interface circuit 23 and likewise the processor 20 can recognise a code embedded in the pic 32 an advantage of using a pic is that just two wires 28 or other conductive paths can be employed to carry the stored data and interrogate the pic via a serial interface and these wires at the tool at least can be used for probe power also additionally the use of two wires means that backward compatibility is possible i e the probe etc previously fitted with a resistor identification as detailed in the discussion of the prior art.