A friend of mine gave me one of those data-gathering bracelets for Christmas. It tells me how far I’ve walked, how long I’ve slept, how fast my heart is beating and whether I’m still alive – usually not far enough, too long, about right and yes, just.

What I wondered, a few days ago, as my mind wandered momentarily during a performance of Wagner’s Tristan and Isolde  (four hours of glorious music, but not much happening on the stage) is whether this magic bracelet, or a future model, might detect the mind wandering.

I do know that mind-reading technology hasn’t yet been invented, but there might be accompanying symptoms such as a low rate of breathing, a declining heart rate, a lack of movement, that together suggest I’m not attending as excitedly as I should.

Then it could administer a mild electric shock..

This might also be a useful feature during interminable PowerPoint presentations.