The magnetic holding power on these ground clamps can be turned on and off so the clamp can be easily positioned. Turning the magnet off also prevents metal shavings and parts from being attracted to the magnet when it's not in use. Magnetic ground clamps attach to any magnetic surface. They're often used on large surfaces where an edge isn't convenient for attaching standard clamps.
Switchable Magnetic Ground Clamps, sorted by Current (Maximum), ascending