The original instrument voltage regulator is a vibrating type. As the current flows through the thin wire wrapped around the arm, it heats it up. Being a bimetallic strip, this causes the arm to bend, breaking contact, which allows it to cool, starting the cycle over again. It does this continuously, with an average voltage of about 6 volts. The only way it can fail is with a short downstream, which causes the fine wire to act as a fuse, burning it in two.