That is the only way that I have seen it done and as long as a car battery charger is used and not the battery itself it is safe. You do still want some space between the sparks and the TOTs.Did this one year in a haunted house with a car battery charger. Hung a large metal screen 9(2'x3') along the back wall and attached the Neg clip of the battery charger for it. Then attached a large metal bolt to the Pos clip of the charger. To make sparks, I just have to swipe the bolt across the metal screen. Don't know how real safe it was, but it was really cool!!! No one got hurt as we kept the guest more than and arms reach from the screen.