Jamieson, Fausset and Brown on Acts 19:13

13. vagabond Jews--simply, "wandering Jews," who went from place to place practicing exorcism, or the art of conjuring evil spirits to depart out of the possessed. That such a power did exist, for some time at least, seems implied in Mt 12:27. But no doubt this would breed imposture; and the present case is very different from that referred to in Lu 9:49, 50.

We adjure you by Jesus whom Paul preacheth--a striking testimony to the power of Christ's name in Paul's mouth.