We were well used to doing this with batteries when children, for fun (you had to make your own amusement back then). One of my mates decided to try it with the output of the 12V transformer we used to run my Triang train set (TT gauge for the anoraks). When he was able to describe the experience, some minutes later, he said it was akin to being hit by two torpedoes, simultaneously, one on each temple. I was disinclined to try it.