Lights out, jammers on!

The lights are dimmed, the hum of voices fades, the curtains onstage sweep open. The moment the spotlight clicks on, the focus of the crowd moves front and centre. Then out of the silence comes a chirpy ring tone from the handbag of a spectator who forgot to switch off her mobile telephone. This is a familiar scenario to most of us. Nearly every day we suffer the minor irritations of people using their cellphones in public: in libraries, on the train, in the supermarket checkout line or at the office.

Thank goodness for the Israeli secret service. Because terrorists can use a mobile telephone as a detonation mechanism—as they did during the Madrid train bombings in 2004—the Israelis developed a jammer. The device blocks the transfer of satellite signals to cell towers, thus disabling mobile telephone communication within a three-mile (five-kilometre) radius.

If this sounds like a drastic measure to ensure an audience can watch a performance undisturbed, rest assured that the technology is only available for military purposes. But there’s a blossoming Internet trade in a variation on the theme. The device (available starting at $170) emits the radio frequency of mobile telephones. The interference blocks calls in a radius of at least 11 yards (10 metres), depending on the strength of the signals.

In the vast majority of places—including Europe and the U.S., but not everywhere in Asia—using a jammer is illegal. Nevertheless, the technique is used in plenty of places. These include several churches in Mexico to ensure respect for the sanctity of worship; at a number of universities in Italy to prevent students from taking photos of tests and sending text messages; in Pakistani banks to keep bank robbers from contacting their accomplices; and inside India’s parliament, where politicians have failed to respect the appeal to switch off their phones. Meanwhile, Japan allows jammers in public places—such as theatres and concert halls—with a permit. And they’re allowed in France as long as a telephone is available for contacting the police or an ambulance.

Despite the ban on jammers, the devices are available via the Internet. Vendors make it clear that their use is illegal. Still, an article in The New York Times late last year reported that overseas exporters send hundreds of them to the U.S. every month. Customers include café owners, hairdressers, hoteliers, therapists, bus drivers—and people who want to enjoy an evening at the theatre.