TELEGRAPH.CO.UK - Mar 5 - Spark is a new dating app that aims to connect people in places such as the Tube. It works both with and without an Internet connection. The app can cleverly function over Bluetooth. When users see someone attractive they open the app, find their profile and send them a Spark. The Tube crush will need to be signed up to the app too, so its usability ultimately depends on how many people create an account. The recipient will only receive the Spark once they have parted ways, saving any potential embarrassment. 4K users signed up before it launched last Sunday. That's still some way behind Tinder's estimated 50M users, but it's a positive start. Although the app was primarily designed with the Tube in mind, it has been rolled out across the UK and is already proving popular in Bristol and Manchester.