How Faraday bags work, and how to tell if yours does

Put your phone in a Faraday bag, roll the top closed and call it from another phone. It goes straight to voicemail. No bars, no GPS, no Wi-Fi. For what is basically a fabric pouch, it can feel like a magic trick.

It isn't magic, though. The idea is almost 200 years old, and once you understand it you'll also know what makes one bag work better than another that looks almost the same.

It started with a room covered in foil

In 1836 the English scientist Michael Faraday lined a room with metal foil and charged the outside of it with high voltage. Inside, his electroscope (an instrument that detects electric charge) didn't move at all. The charge stayed on the outside of the metal and never reached the inside of the room.

That kind of enclosure has been called a Faraday cage ever since, and you've almost certainly been inside one. It's the reason the metal mesh in a microwave door keeps the microwaves in, why calls tend to drop in elevators, and why people inside a plane are fine when it gets hit by lightning.

A Faraday bag is the same thing in a form you can fold up and carry. The walls are flexible fabric woven with metal fibers, usually copper and nickel, instead of rigid sheets of metal.

What actually happens to the signal

Every wireless thing your phone does, from calls and texts to GPS, Wi-Fi and Bluetooth, uses radio waves. Radio waves are the same basic stuff as visible light, electric and magnetic fields moving together at the speed of light. They just vibrate much more slowly, so you can't see them.

Metal is full of free electrons, tiny charged particles that can move around easily. That's what makes metal a good conductor. When a radio wave hits the lining of the bag, it pushes those electrons back and forth in time with the signal, and the moving electrons create a field of their own that pushes against the incoming one. On the inside of the bag, the two more or less cancel each other out.

In practice most of the signal bounces off the outside of the bag. What does get into the metal fades out as a trace of heat far too small to notice. A bag with two or three layers of shielding repeats the process, so anything that sneaks past the first layer gets stopped by the next.

It also works in both directions, which is easy to forget. Nothing gets in, and your phone's own signal doesn't get out either. To the cell network, your phone has simply disappeared.

Why a bag can leak even when the fabric is fine

A Faraday cage doesn't have to be solid metal. A mesh works just as well, as long as the holes are much smaller than the wavelength of the signal you're trying to stop. The shorter the wavelength, the smaller the gap it can squeeze through, which is why newer, higher-frequency signals are the hardest to block.

Good shielding fabric is woven so tightly that its gaps are microscopic, far smaller than even the 1 cm waves at the bottom of that table. So when a bag lets a call through, the fabric is rarely the reason. It's usually one of these:

• The opening. A zipper or a single fold leaves a slot that signals can leak through. That's why better bags use a roll-top or a double fold-over closure, so the shielding layers overlap before they're sealed.
• The seams. Every stitch is a tiny hole through the metal layer. Well-made bags overlap or seal their seams so there's no straight path through.
• Wear. Folding the fabric sharply in the same spot hundreds of times can crack the metal coating and open up small gaps.

When a Faraday bag lets a call through, it's almost always the closure, not the fabric.

How much blocking is enough?

Shielding is measured in decibels (dB) of attenuation, which tells you how much weaker the signal is inside the bag than outside it. The scale is logarithmic, so every extra 10 dB makes the signal ten times weaker again.

A "99% blocking" claim sounds impressive, but that's only 20 dB, and phones are very good at holding on to weak signals. If you want a phone to go completely silent, look for 60 dB or more across the whole range of frequencies above, not just one or two of them.

What people actually use them for

• Keyless car keys. Relay theft uses two radios to pass your fob's signal from inside your house to your car, so thieves can unlock and start it from the driveway. A fob in a sealed pouch can't be picked up.
• Keeping a phone private. With no cellular, Wi-Fi, Bluetooth or GPS connection, your phone can't report where it is to apps, networks or anyone else.
• Contactless cards and passports. The chips in them can be read by a scanner held close enough. Shielding stops the reader from powering the chip in the first place.
• Evidence and remote access. A phone that can't connect can't be wiped or controlled remotely. Police and forensic teams bag seized phones for exactly this reason.
• Hardware wallets and spare devices. Crypto wallets, backup phones and drives stay completely offline while they're stored.
• EMP and solar storm prep. A well-sealed, multi-layer bag can help protect small electronics from electromagnetic surges. If you're serious about it, nest two bags or use a sealed metal box as well.

What a Faraday bag won't do

• It only works while it's closed. Take the phone out and it reconnects and syncs straight away.
• It can't undo anything. Location history, synced data or malware that's already on a device stays there. The bag just stops new connections.
• You can't use your phone while it's inside. No calls, texts or notifications. That's the point.
• It isn't meant for very low-frequency magnetic fields, like the ones around power lines and household wiring. Those need a different kind of shielding.
• A half-closed bag barely works. Even a small gap at the top can let a strong signal in.

Test yours in a couple of minutes

You don't need any equipment beyond a second phone. It's worth doing when a bag is new and again every few months.

1. Call test. Seal your phone in the bag and call it. It should go straight to voicemail. Try again standing next to a window, where the signal is usually strongest.
2. Bluetooth test. Play music from your phone to a Bluetooth speaker, then seal the phone in the bag. The music should stop within a few seconds.
3. Wi-Fi test. Stand next to your router, start a video on the phone and seal it in. The video should freeze almost immediately.
4. Key fob test. Put your car key in the pouch, walk up to the car and try the door handle. It shouldn't unlock.

Looking after it

• Close it fully every time, using all the folds or rolls the closure is designed for.
• Try not to crease the lining sharply in the same place over and over.
• Keep keys, pens and anything sharp away from the inner lining.
• Check the lining for tears or worn patches now and then, and redo the call test.
• Switch devices off if you're storing them for a while. A phone that keeps searching for a signal drains its battery faster.