Current loops...

Just a touch more on top of what Arne said.....
Instrumentation - 4-20 mA, 0-4mA and 1-5mA were all in quite
common use, with the latter being use by some vendors as a
proprietary wrench on locked in customers. 'nuff on that.

20mA and 60mA were both common in TTY loops, with TELEX using
both plus reversal of polarity (to indicate "off hook" sorta)
in the "loop" protocol.

Normally, +130v to ground was used for TTY, but for extra range,
+/- 130vdc could be used. (to overcome long loop resistance and
still have enough current flow). 20mA was not as immune to
noise as 60mA....

+130vDC was often wired to places it wasn't desired, like to
some conductors in a brand new VOICE cable in a central office,
where an installer was terminating said cable on a dist. frame.,
pliers on the wire, one hand on the ironwork. Nasty shock.
But we won't go there..... / mark