Armored Cable (AC) not air conditioning
Conductors protected by a spiral-wound outer metal sheathing have been around since the early part of the century, and they gained wide acceptance in the 1930s, especially after the NEC's acceptance in the 1932 Code.
Several types of AC cable exist and they are not all the same. The earliest type was introduced by General Electric under their brand name "BX." Many people still wrongly call all type-AC cables by this name.
Type-AC cables fall into two categories: those with an internal bonding conductor and those without. In many cases, the sheathing itself, or its internal bond, has been used improperly as the grounding conductor, or, even worse, as the neutral conductor.
As of 1959, the NEC has required that all type-AC cable includes a bonding strip which connects all the individual convolutions. The older "BX" cable did not have this, and the exterior metal casing was not meant to be an effective fault current path. Since the 1960s, a newer type of AC-cable assembly came onto the market. The improved MC cable includes a proper grounding conductor. Thank you InterNachi