Variable or adjustable (pre-set) capacitors

85.32 - Electrical capacitors, fixed, variable or adjustable @re-set) (+). 8532.10 - Fixed capacitors desiped for use in 50/60Hz circuits and having a reactive power handling capacity of not less than 0.5 kvar (power capacitors) - Other fixed capacitors : 8532.21 - - Tantalum 8532.22 - - Aluminium electrolytic 8532.23 - - Ceramic dielectric, single layer 8532.24 - - Ceramic dielectric, multilayer 8532.25 - - Dielectric of paper or plastics 8532.29 - - Other 8532.30 - Variable or adjustable (pre-set) capacitors 8532.90 - Parts Electrical capacitors (or condensers) consist, in principle, of two conducting surfaces se arated by an insulating material (dielectric), e.g., air, paper, mica, oil, resins, rubber and p astics, ceramics or glass. They are used for various pufposes in many branches of the electrical industry ( e p . to improve the power factor of AC circuits, to produce base-shifted currents for rotating fie1 s in induchon motors, to protect electrical contacts from %e effects of arcing, for storing and releasing given quantities of electricity, in oscillatin circuits, in frequency filters, and very widely in the radio, television or telephone industries or or industrial electronic equipment). !? Their characteristics (shape, size, capacitance, nature of dielectric, etc.) vary accordin to their intended use. The heading, however, covers all capacitors irrespective of their type an method of manufacture and whatever their intended use (including standard capacitors used in laboratories or in numerous measuring instruments, specially made within fine limits and designed to remain constant during use). % The heading also covers capacitors grouped together on a chassis or in a container (e.g., certain lar e power factor capacitors and capacitor boxes consisting of a number of standard capacitors wit8, means of connecting them together, in series or in parallel, to obtain any required capacitance). (A) FIXED CAPACITORS Fixed capacitors are those in which the capacitance cannot be varied. The main types are : dry capacitors, " oil " imprepated capacitors, " gas impregnated capacitors, " oil " filled capacitors and electrolytic capacitors. " (1) In dry capacitors the plates and dielectric are usually in the form of superimposed plates or rolled stnp or foil. In certain dry capacitors a metal covering is applled by a chemical or thermic process to a fixed dielectric. Dry capacitors may be enclosed in a clamp-equipped box or be used without a box. (2) " Oil " impregnated ca acitors are similar to dry capacitors but the dielectric, usually of plastic film or plastic and paper, is impregnated with oil or other liquid. & (3) "Gas " irn regnated capacitors are capacitors consisting of two or more electrodes separated I? y a gas, other than air, which serves as a dielectric. (4) In some cases the capacitor (" oil filled capacitor ") is enclosed in a container filled with oil or other suitable liquid, and many incorporate accessory devices such as pressure gauges and safety valves. (5) In electrolytic capacitors, one of the plates is generally of aluminium or tantalum while the role of the other is fulfilled by a suitable electrolyte to which the current is led by means of an electrode, sometimes similar in form to the aluminium or tantalum plate. The electrolyhc action produces on the aluminium or tantalum a thin layer of complex chemical compounds which thereafter constitutes the dielectric. The capacitor is sometimes enclosed in a container, but enerally the outer electrode itself constitutes the container; these capacitors are often itted with a in base similar to that of a valve. Certain capacitors of this type containing a pasty o m of electrolyte are also called "dry electrolytic capac~tors". k P (E3) VARIABLE CAPACITORS Variable capacitors are those in which the ca acitance can be varied at will. In most cases air is the dielectric, and the plates usually consist o two groups of metal plates, one group fixed while the other, mounted on an axis, can be turned so that its plates can pass between the fixed plates. The de ee to which the mobile lates (rotors) are turned and overlap with the fixed plates (stators$varies the capacitance of tie capacitor. f (C) PRE-SET OR ADJUSTABLE CAPACITORS Pre-set or adjustable capacitors (including trimming capacitors) are those in which the capacitance can be adjusted within narrow limits to a precise value. This adjustment can be made in different ways. In certain types the distance between the plates may be varied by means of a screw. Other es consist of two metal cylinders, one of which can be moved to a variable extent within the o er or of two mutually moved semicircles. Usually, the dielectrics used are, for example, mica, ceramics, plastics or ar. % PARTS Sub'ect to the general provisions regarding the classification of arts (see the General Exp anatory Note to Section XVI),parts of capacitors are also classified ere. % The heading does not cover certain synchronous motors used for power factor improvement, even though they are often called " synchronous capacitors " (heading 85.01).