Politicians and law enforcement officials in California will introduce a bill on Friday that requires all smartphones and tablet PCs sold in the state be equipped with a digital "kill-switch" that would make the devices useless if stolen. The bill is a response to a rise in thefts of portable electronics devices, often at knife or gunpoint, being seen across the state. Already half of all robberies in San Francisco and 75 percent of those in Oakland involve a mobile device and the number is rising in Los Angeles, according to police figures.