Toy Shop by Harry Harrison, Science Fiction, Adventure

The gadget was strictly, beyond any question, a toy. Not a real, workable device. Except for the way it could work under a man's mental skin. . . .

Because there were few adults in the crowd, and Colonel "Biff" Hawton stood over six feet tall, he could see every detail of the demonstration. The children -- and most of the parents -- gaped in wide-eyed wonder. Biff Hawton was too sophisticated to be awed. He stayed on because he wanted to find out what the trick was that made the gadget work.