Olin Lathrop wrote:

>> Even better if the swich is organised to provide the constant current
>> source say 4mA "closed", 20mA "open".
> 
> Um, switches tend to conduct a lot better closed than open.

Um, yes, of course :)  

But as I understand this approach, this is really not the point. The switch
wouldn't be connected to the wires...

Voltage Source >--------long wire-------------------+
                                                    |
                                              --------------
                                              | Controlled |----+
                                              | Current    |  Switch
                                              | Source     |----+
                                              --------------
Protected                                           |
Voltage  <--+-----------long wire-------------------+
Input       |
            R
            |
           Gnd

So whether it's easier to have 4 mA closed or 4 mA open depends a lot on
how you make the circuit in the current source. There are probably quite a
number of alternatives.

Gerhard
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