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80 such as paraffined paper. The object of using some lining between the plates is twofold. In the first place it would be technically very difficult to insure a very small intervening space without the risk that the plates come actually into contact. If the condenser is not required to have a large capacity, and especially if it is to be used as a standard of capacity for comparison with other condensers, then the intervening space between the plates may be left without filling material. Such condensers are called "air condensers." Where a condenser of larger capacity is required as a standard, then the filling-in material, the so-called "dielectric," may be mica. This, even in thin sheets, is electrically very strong, and is also an excellent insulator. It is thus possible to make the space between the plates very small, and by this means obtain a larger capacity with a given plate surface than with an air condenser. The other reason for using another material than air as a dielectric is that the material by itself has the property of increasing the capacity.

We have seen in Chapter I that the attractive force depends on the medium between the two charged surfaces. If this medium is air, the force is greatest; if it is an insulator such