Weird voltage

Installing a new fan in an old house. 2 wires come thru the ceiling. 1 romex goes to the switch, the other is the hot feed 2 wire asphalt wire in bx cable. Hooked one of the bx to one side of the switch wire. next I tested the remaining wires one from the bx and the return from the switch. turn on the switch get 110v, but when I turn the switch off and test the wires I get voltage (80v) weird. Switch the feed wires and get the same results.