Faraday cages

Last month I mentioned how building materials can block cell signal by acting as a Faraday cage. The Faraday cage phenomenon is a pretty interesting one and I thought it deserved a bit more attention.

A Faraday shield consists of a wire mesh which blocks external electric charges from entering the cage. The video below, from a German university, illustrates its purpose pretty vividly.

In addition to stopping the person inside from being electrocuted, the Faraday cage blocks electromagnetic fields and frequency waves – the latter resulting in interrupted radio and cell coverage. When such waves encounter conductive material, they do not pass through the material but rather flow around its surface. For example, when lightning strikes the metal frame of a car or airplane, that frame shields the passengers inside from the electric charge.

Faraday cages, intentional or otherwise, are everywhere in our daily lives. Some auditoriums and exam halls use them to prevent disruption or cheating in tests. Microwave ovens have inbuilt Faraday cages to contain the microwaves within them. In practice, we expect to encounter them regularly, anticipating losing radio signal driving under a bridge, or cell signal when entering an elevator.

Most homes were certainly not deliberately constructed as Faraday cages. For one thing, a true Faraday cage would be exceptionally expensive, and only of interest to sensitive government buildings. Nevertheless, the wire frames and meshes used in building construction have the effect of blocking or severely reducing the quality of cellular signals inside them. This is obviated somewhat by a building’s doors and windows, but the ‘Faraday cage effect’ still results in many, many homes and offices receiving little or no cell phone coverage.

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