Read a Lamp Cord's Polarity by Feel: Ribbed Is Neutral, Smooth Is Hot

The Problem

Rewire a lamp or swap a plug and you meet zip cord: two conductors, both insulated in the same brown or white jacket, no black-and-white color coding anywhere. It looks like it can't matter which wire goes where. It does — a lamp wired backward still lights, but the socket's threaded shell and the switch end up on the hot side, which means the exposed metal a fingertip can brush while changing a bulb is live.

The Technique

The cord is marked — by texture, not color:

1. Run a fingernail across both conductors' jackets, or look closely under good light.
2. One side has fine ribs or ridges molded along its length; the other is smooth.
3. Ribbed is neutral. It connects to the wide prong of the plug and the silver screw — on a lamp socket, that's the threaded shell.
4. Smooth is hot. It connects to the narrow prong and the brass screw — the socket's center contact, through the switch.

Wired that way, the switch interrupts the hot, and the threaded shell around the bulb sits at neutral — the arrangement every polarized plug exists to protect.