Hand Held Gadget
A few decades ago mobile telephones were the stuff of science fiction, like the communicators used in Star Trek, but since February 2009 there have been more than 4 billion subscriptions to mobile phone services. Mobile phones are globally ubiquitous.
mobile phone pictured left was used for 3 different functions; as a
telephone, as a pocket watch and as an alarm clock. So effectively it
replaced the items on the right.
Today this mobile is used as a torch, telephone, pocket watch, alarm clock, music player (mostly podcasts), sat' nav, camera, radio, notebook, address book, calendar and calculator.
Smartphones have combined the functions of a PDA and a mobile phone. The vision of this video , a portable device that can also be used as a desktop computer, was being followed by Windows 10 Continuum and is being followed by Samsung DeX which could, possibly, be seen as the future of personal computing.
A mobile is obviously very useful. It has many functions, those functions are available whenever its to hand (for some functions a 'signal' is needed too) and the burdens are fairly low - the purchase cost, keeping it charged and the cost of the monthly bill.
Usefulness = (Functionality x Availability) – Burden