I thought this was a joke when I first saw it but some investigation leads
me to believe that it is real:

http://www.ohm-labs.com/resistance-standards/100-0-standard.html

It seems that this device (a four-terminal device where the voltage between
the sense terminals is zero regardless of the amount of current flow
between the other pair) was originally developed to allow series and
parallel combinations of very accurate four-terminal resistors, producing
an overall four terminal resistor with the ideal series or parallel
combination resistance of the four-terminal resistors that comprise it.

What I don't understand, though, is its utility as a stand-alone item. You
could "do what it says on the tin" by just completely isolating the sense
terminals from the force terminals and shorting the sense terminals
together and the force terminals together. The only difference I can see is
that there would not be conductivity between the force terminals and sense
terminals and so therefore there could be a voltage difference between the
two pairs. But then again it would seem to me that you could achieve the
same effect with simply a block of copper where the force terminals were
placed anywhere on the block but close to each other, while the sense
terminals were placed far away, on the same block, from the force
terminals, but also close to each other.

Inside this "zero ohm standard resistor" is indeed a thick copper disk
(from what I've read) where the terminals are located in a very symmetrical
configuration so that the voltage drop due to the force terminals has
exactly the same value at the two places where the sense terminals connect.

Sean
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.