What's behind your bedroom wall
A wall is not a blank surface. It is a utility corridor with drywall over it, and the meter can read the traffic.
The traffic inside a typical wall
Open up any interior wall in a house and you find the same cast of characters. Branch circuit cables looping between outlets and switches. Sometimes a thicker feeder heading to a subpanel or an electric range. Supply and drain plumbing if a bathroom or kitchen shares the wall. Ducts, gas lines, and in newer houses, low-voltage bundles for data. Most of it produces no measurable magnetic field at all, because as explained on the net currents page, a properly wired cable carries its outgoing and returning current side by side and the fields cancel.
The interesting cases are the ones where cancellation fails or where sheer proximity defeats it:
| What's in the wall | What the meter shows |
|---|---|
| Ordinary branch circuits, wired correctly | Near nothing beyond a few inches, even under load. |
| The wall behind or beside the service panel | A hot zone a few feet wide where every circuit in the house converges. Panels belong behind garages and closets, but plenty of bedrooms share a wall with one. |
| A plumbing wall carrying net current | Elevated readings along the whole wall, strongest near the pipe run, persisting with your main breaker off. |
| A circuit with a wiring error (shared neutrals, stray neutral-ground bond) | A readable stripe that follows the cable path and switches on and off with a specific breaker. |
| Knob-and-tube wiring in old houses | The rare case where correct wiring still radiates: the hot and neutral run separated by design, so the loop between them radiates under load. |
How to read a wall
Sweep the meter slowly across the surface at a constant few inches of standoff, then again at arm's length. A stripe that fades fast with distance is a cable close to the surface, usually harmless a foot away. A broad zone that barely fades is either a lot of conductors together (panel wall) or net current (plumbing, or a miswired circuit). Then run the breaker experiment: kill circuits one at a time and watch which one owns the reading. Anything that survives the main breaker belongs to the plumbing or the street, not to your wiring.
Do this before deciding where furniture goes and the information is free forever after. It is worth reading the walls of a child's room even when nothing seems wrong, because the cost is five minutes, and the finding, when there is one, is usually a wiring defect worth fixing for its own sake.
When you open a wall, look
Renovations are the one time the corridor is visible. If a remodel has the drywall off anywhere near where people sleep, that is the moment to have the electrician correct anything questionable: stray neutral-ground connections, junction boxes where neutrals of different circuits got tied together, cables that could be re-routed away from a bed location for the cost of a few staples. Wiring corrections that would be a four-figure exploratory job through closed walls become trivial line items while the studs are exposed. The same logic applies to any built-in project that opens bedroom walls, which is a subject the next article in this series takes up in detail.
The reassuring baseline
Most walls read clean. The point of learning to read them is not that houses are dangerous; it is that the few real findings hide behind identical drywall, and no amount of looking at a wall tells you which kind you have. The meter does, in minutes, and then the fixes are usually the cheap, permanent, source-side kind this site exists to describe.