Editing Current loop (section)

==Long circuits==
Analog current loops were historically occasionally carried between buildings by [[dry pair]]s in telephone cables leased from the local telephone company. 4–20&nbsp;mA loops were more common in the days of analog telephony. These circuits require end-to-end direct current (DC) continuity, and unless a dedicated wire pair was hardwired, their use ceased with the introduction of semiconductor switching. DC continuity is not available over a microwave radio, optical fibre, or a multiplexed telephone circuit connection. 
Basic DC circuit theory shows that the current is the same all along the line. It was common to see 4–20&nbsp;mA circuits that had loop lengths in miles or circuits working over telephone cable pairs that were longer than ten thousand feet end-to-end. There are still legacy systems in place using this technology. In [[Bell System]] circuits, voltages up to 125&nbsp;VDC were employed.