of an accurately centred permanent magnet face. Electromagnets or permanent magnets are brought into contact with fixed ferrous plates, or pole pieces, contained within a housing. These pole pieces are usually flush with the housing surface. The part (workpiece) to be held forms the closing of the magnetic loop or path, onto those fixed plates, providing a secure anchor for the workpiece. Electrostatic Commonly used for holding silicon wafers during lithography processes, an electrostatic chuck comprises a metal base-plate and a thin dielectric layer; the metal base-plate is maintained at a high-voltage relative to the wafer, and so an electrostatic force clamps the wafer to it. Electrostatic chucks may have pins, or mesas, the height of which is included in the reported dielectric thickness; a design by Sandia National Laboratory uses a patterned silicon-dioxide dielectric to form the pins. Vacuum A vacuum chuck is primarily used on non-ferrous materials, such as copper, bronze, aluminium, titanium, plastics, and stone. In a vacuum chuck, air is pumped from a cavity behind the workpiece, and atmospheric pressure provides the holding force. Vacuum produces a hold down pressure of at sea level, decreasing at higher elevations where the atmospheric pressure is lower. The decrease in holding pressure is roughly 0.5 psi per 1000' above sea level. Mounting methods Connecting chucks to the spindles or tables of machine tools or power tools has been accomplished in many ways over the years. Mounting of drill chucks A may screw into the chuck body. A (with a self-holding taper) may be pressed into the chuck body. Removal and insertion may involve various tools or methods: Tools: vise (especially with a wooden jig or soft jaw made for this purpose); hammer (especially nonmarring hammer or rubber mallet); arbor press or shop press (the latter two require skill to avoid damaging the chuck). Methods: heating (via heat guns, blow torches, or ovens) and cooling (via kitchen freezers, winter weather, or decompression of compressed air or nitrogen). A drill chuck may have a hollow body that threads directly onto a lathe's threaded spindle nose. (These are fairly rare, especially nowadays.) Mounting of large jawed chucks A backplate with threads may screw onto a threaded spindle nose (for lathe work) or onto an adapter plate with the same nose, to be mounted on the table of milling machines or surface grinding machines. This "threaded spindle nose" type of mounting was the