FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION (See Section 10 and Rule 13) 1. Title of the Invention:- AN APPARATUS AND A METHOD OF CONTROLLING GLASS SHEET THICKNESS 2. Applicant(s):- (a) Name: STERLITE TECHNOLOGIES LTD. (b) Nationality: An Indian Company (c) Address: E1/E2/E3, MIDC, Waluj, Aurangabad - 431136 Maharashtra, INDIA 3.preamble to the description: - Complete Specification: The following specification particularly describes the invention and the manner in which it is to be performed. AN APPARATUS AND A METHOD OF CONTROLLING GLASS SHEET THICKNESS FIELD OF THE INVENTION [0001] Embodiments herein generally relate to manufacturing of glass sheets by an overflow downdraw process and in particular to an apparatus and a method of controlling of glass sheet thickness using the overflow downdraw process. BACKGROUND OF THE INVENTION [0002] Devices such as the mobiles or cell-phones, palm top computers, watches, laptops, portable gaming devices with displays, notebooks, displays in vehicles, touch panels screens, televisions (with flat display panels) and other electronic devices etc., have become ubiquitous. A glass display screen forms an essential part of these devices. The glass display screen may be a passive display or an active display (as in case of touch screen displays). Semiconductor material is deposited on a glass sheet of predetermined thickness and flat surface to achieve the colors and/or various other display characteristics. [0003] It is known that the glass sheet that is used in above-mentioned devices, on which the semiconductor material is deposited, must have very high surface quality to allow the successful application of semiconductor type material. [0004] For example, the U. S. Pat. No. 3,338,696 [referred to as US '696 hereinafter] discloses an apparatus for forming sheet glasss using the overflow downdraw process. These overflow downdraw process along with the apparatus disclosed in US '696 is known to make high quality glass sheets as formed and does not require any post-processing. The most important aspect of the overflow process is that the glass moves through the glass forming equipment and the untouched, "virgin glass", overflows and becomes the outside surface of the glass. Glass made using other processes requires grinding and/or polishing and thus does not have as fine a surface finish. [0005] FIG. 1 depicts an apparatus 100 in accordance with US '696, wherein a feed pipe 101 supplies molten glass through an opening 102 to a flow passage 103 of a sheet forming trough or weir device 104. The molten glass flows into a contoured upwardly-open channel 105 which regulates the molten glass flow such that the flow remains uniform. The channel 105 is bounded by vertical interior sidewall 106 and 107 which are substantially perpendicular to a contoured bottom surface 108 forming the bottom of channel 105. [0006] In accordance with US '696, the bottom surface 108 is inclined at an angle such that at the farthest end from the feed or flow passage 103, the bottom surface 108 touches the upper edge of the sidewall 106 and 107. The channel portion of the weir device is so contoured and designed as to enable the production of glass sheet material with a uniform desired cross section along it width. [0007] FIG. 2 illustrates a cross sectional view of the forming apparatus 104 having a triangular shaped body portion 109 with two walls or surfaces 110 and 111 oppositely disposed. The channel 105 with the contoured bottom surface 108 is longitudinally located in the blunt upper surface of the wedge shaped body portion 109. The trough or weir 104 may be pivotally adjusted by any suitable means such as an adjustable roller, wedge or cam 112 to provide desired tilt angle φ which is the angular variation from the horizontal of parallel upper edges or surfaces 113a and 114a on sidewalls 106 and 107. The parallel upper edges may be made with removable bars or plates or dams 113 and 114. The dams 113 and 114 can be removed, resurfaced and reinserted while the trough is hot. The molten glass 115 entering the channel 105 is allowed to flow over the dams or weirs 113 and 114, then over the converging walls 110 and 111 to meet at a point 116, generally referred to as root, resulting in formation of a sheet of glass 117. [0008] The thickness of the glass sheet 117 being formed should be controlled such that it remains uniform across the longitudinal axis of the forming apparatus 104 as well as over the length of the sheet. The thickness of the glass sheet 117 if not controlled may lead to distortion of the image or picture quality in the display device where it is used which is not desired. [0009] The apparatus of US '696 as disclosed provide a way out by tilting the forming apparatus 104 at an angle φ to control thickness of the glass sheet being formed. [00010] However, the apparatus of US '696 have limitations. It has been observed that a glass sheet of uniform thickness cannot be obtained using the apparatus as disclosed in US '696. In practice the angle of tilt has practical limitations and the simple linear geometry of the apparatus of US '696 fails because of non-linearity of the glass flows and that of the glass viscosity. [00011] Further, the apparatus provides a limited range of production rates that can be accommodated by a given apparatus that is if the flow rates or viscosity of the glass is changed then the same apparatus may not be used for forming the glass sheet. [00012] Still further, over the period of time, the forming apparatus are subjected to changes in form or shape originating due to wearing down of material and by thermal creep of the material from which the apparatus are built. The US '696 does not provide, any provision for correcting the curvature or other non-uniformity in the thickness of the glass sheet owing to these effects. [00013] Another US patent no. 6,748,765 [referred to as US '765 hereinafter] discloses forming apparatus design features that may be used alone or in combination to facilitate faster, more uniform flow of glass through the apparatus and allows the thickness of the sheet to be adjusted. [00014] In accordance with US '765, the forming apparatus is provided at the far end of the trough [end away from the feed pipe] an over flow apparatus which is used in conjunction with tilting of the apparatus to expand the range of glass flow rate and glass viscosity for which this invention will produce satisfactory product. [00015] Further in accordance US '765, the forming apparatus can be made with non-linear weirs [or dams] and the trough bottom to provide a greater range of flow rates. [00016] In accordance with some embodiments of US '765 the glass flow in the inflow pipe can modulated or a contoured trough cross-section used to provide more uniform time dependent flow for forming the sheet. [00017] However, it is to be noted that though US 765 though discloses the use of "non-linear" weirs to control the glass flow over the weirs [or dams] to achieve uniform glass sheet thickness in addition to other advantages, the specification of US '765 is silent about the exact shape of the weirs that would lead to uniform glass sheet thickness. Some exemplary drawings do refer to possible parabolic or arc shaped weirs. [00018] Further, it may be noted that the term "non-linear" is too broad and encompasses very other shape than linear one. However the inventors of the present invention have observed that US '765 do not even provide "computational" and/or "experimental" data to substantiate their claims. [00019] The US '765 also discloses some other measures to achieve the uniform glass sheet thickness objective such as modulating the molten glass flow in the flow pipe, changing the shape of glass flow pipe inlet, the bottom trough shape etc., which are difficult to fabricate, maintain and/or control. [00020] The Japanese patent application no. 2006-213579 [referred to as JP '579 hereinafter] discloses an apparatus for forming "plate glass" [glass sheet] whose forming capacity is improved and which can keep the stable plate thickness-forming dimensional grade for a long period of time. A method of forming the plate glass using the plate glass forming apparatus is also disclosed. [00021] In accordance with JP '579 the apparatus for forming the plate glass has a gutter-like molten glass supply groove [which is similar to channel 105] having an open upper part on the top part, both side wall top parts of the glass supply groove are used as an overflow weir and both side walls are terminated at a lower end so that the cross-section of the outside surface parts of both side walls is formed to be almost wedge-shaped. Further, the apparatus for forming the plate glass, the molten glass is continuously supplied from one end of the glass supply groove, is over-flown from both side-wall top ridges and is made to flow down along both side wall outside surface and is joined at the wedge-shaped lower end. [00022] In accordance with JP '579 the overflow weirs are parabolic in shape with highest height at the center of the forming apparatus and lowest at both the ends of the forming apparatus [longitudinally]. [00023] It is observed that the forming apparatus of JP '579 does provide correction to the glass sheet thickness however the correction is limited to a narrow range of flow rates of molten glass. [00024] Thus, from the above discussion it is evident that using the downdraw process [also known as the fusion technique] in accordance with the known prior art it is not possible to achieve a glass sheet with uniform thickness along its length and width without having a complicated setup as described in herein above. [00025] Therefore, there is a need to have a forming apparatus which would be simple to fabricate, maintain and operate as well. OBJECTS OF THE PRESENT INVENTION [00026] Accordingly, an object is to provide a forming apparatus. [00027] Another object is to provide a forming apparatus for manufacturing glass sheets with uniform thickness. [00028] Still another object is to provide a forming apparatus that is simple to fabricate, maintain and operate. [00029] Yet another object is to provide a forming apparatus for manufacturing glass sheets with uniform thickness along its length and width. [00030] Another object is to provide a forming apparatus for manufacturing glass sheets for a range of glass flow rates and viscosities. [00031] Other objects and advantages of the present invention will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present invention. SUMMARY OF THE INVENTION [00032] The present invention provides an apparatus and a method for controlling glass sheet thickness using an overflow downdraw process. [00033] In accordance with the present invention, the apparatus for forming sheet glass comprises a body member having sidewalls along with a downwardly converging triangular portion, the sidewalls terminate at their upper extent in top surfaces forming an upwardly open trough bounded by the sidewalls. The triangular portion has two surfaces that meet at a point called root. A feed pipe that is connected to an in communication with an opening and a passage supplies molten glass from a reservoir to the upwardly open trough. The upwardly open trough further has a contour bottom surface for providing a uniform height of molten glass overflowing wherein the contour bottom surface is featured in that it has a progressively decreasing height of such trough as it extends outwardly from the feed pipe. The top surfaces of the sidewalls are provided with weirs or dams that substantially stretch along the length of the top surfaces, wherein the improvement in the apparatus comprises in that the shapes of the weirs is a combination of at least three shapes to facilitate the production of sheet glass having improved thickness uniformity across its width. [00034] In accordance with one embodiment of the present invention the apparatus comprises a cam for allowing inclination of the apparatus as a whole to a desired angle. [00035] In accordance with one embodiment of the present invention, the shape of the weirs is a combination of parabola, straight line, zig-zag line and complex curved line. [00036] In accordance with one embodiment of the present invention a method of controlling thickness of a glass sheet being formed is provided wherein the method comprises the steps of providing an apparatus for forming sheet glass as described hereinabove, providing the top surfaces of the sidewalls with weirs or dams that substantially stretch along the length of the top surfaces, wherein the shape of the weirs is a combination of at least three shapes allowing molten glass to flow over the weirs; and forming said molten glass into sheet 317 having improved thickness uniformity across its width. BRIEF DESCRIPTION OF THE FIGURES [00037] FIG. 1 illustrates a forming apparatus in accordance with the prior art. [00038] FIG. 2 illustrates a cross sectional view of the forming apparatus in accordance with the prior art. [00039] FIG. 3a illustrates a forming apparatus in accordance with exemplary embodiments of the present invention. [00040] FIG. 3b illustrates a cross sectional view of the forming apparatus in accordance with the embodiments of the present invention. [00041] FIG. 4a is a graph of variation of thickness of glass sheet along the length of the weirs in accordance with the prior art. [00042] FIG. 4b is a shape of weirs along the length of the weirs in accordance with an exemplary embodiment of the present invention. [00043] FIG. 4c is a graph of variation of thickness of glass sheet along the length of the weirs for the shape of weirs illustrated in FIG. 4b in accordance with exemplary embodiment of the present invention. DETAILED DESCRIPTION OF THE PRESENT INVENTION [00044] The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those skilled in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein. [00045] The present invention herein generally relate to manufacturing of glass sheets formed by an overflow process and in particular to an apparatus and a method of controlling of glass sheet thickness during the forming process. [00046] In accordance with the embodiments of the present invention a forming apparatus is disclosed wherein a feed pipe supplies molten glass through an opening to a flow passage of a sheet forming trough or weir device. The molten glass flows into a contoured upwardly-open channel which regulates the molten glass flow such that the flow remains uniform. The channel is bounded by vertical interior sidewall and which are substantially perpendicular to a contoured bottom surface forming the bottom of channel. The forming apparatus has a triangular shaped body portion with two walls or surfaces oppositely disposed. The channel with the contoured bottom surface is longitudinally located in the blunt upper surface of the wedge shaped body portion. The trough or weir may be pivotally adjusted by any suitable means such as an adjustable roller, wedge or cam to provide desired tilt angle