Electricity for public schools and colleges . al. The oppositepairs of these are connected. By u is represented a light aluminium needle suspended bytwo parallel silk fibres—this being called bi-filar suspension. Hence there will be called into play, when the needle isdeflected from its position of rest, a couple tending to restore value of this couple for any given angle of deflexion can becalculated. This needle swings horizontally in the interior of thesectors ; and matters must be so arranged that it may come torest exactly along the line of one of the slits. The diagram repre-sents
Electricity for public schools and colleges . al. The oppositepairs of these are connected. By u is represented a light aluminium needle suspended bytwo parallel silk fibres—this being called bi-filar suspension. Hence there will be called into play, when the needle isdeflected from its position of rest, a couple tending to restore value of this couple for any given angle of deflexion can becalculated. This needle swings horizontally in the interior of thesectors ; and matters must be so arranged that it may come torest exactly along the line of one of the slits. The diagram repre-sents the needle thus at rest, unacted upon by any electricalforces. This needle is maintained at some constant potential V, bybeing connected with the inside coating of a charged Leyden jar. If the one pair of sectors c and b be at a potential Vj, andthe other pair a and d at some lower potential Vo, there will bea field of force running across the gap or slit from c io a ; andfrom symmetry, an equftl field will run across the gap from b to Fig. 155 ELECTRICITY CH. X. Moreover, this field will be confined to a region only somewhatwider than these gaps ; the region in the interior of each sectorwill be practically of a uniform potential, or will not be a field offorce. From the symmetry of the whole, the needle will be actedupon by a pure couple due to the electrical field. And, from itspeculiar shape, that amount of the needle which is in the field offorce { the portion lying under the slit) will remain constant,since only very small deflexions are employed. We thus contrivethat there shall act on the needle, for any given values of V,, V,„and V, a constant electrostatic couple whatever be its deflexion ;provided that this deflexion do not exceed a certain maximumdepending on the construction of the instrument. This constant couple will deflect the needle until the restoringcouple, due to the twisting of the two paraUel silk fibres, is suffi-ciently great to give equili
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Keywords: ., bookcentury1800, bookdecade1890, bookpubl, booksubjectelectricity
