Saved Bookmarks
| 1. |
DescribeKelvin'smethod to determinedthe resistance ofgalvanometerby usingmeterbridge . |
|
Answer» Solution :Kelvin's method of determinethe resistanceof the galvonometer by using a meter bridge : A galvanometer whose resistanceG is to be determinedis connected in onegap (left gap ) of aWheatstone's meter bridgeand a resistancebox isconnectedin the other gap (right gap).  G : Galvanometer R : Resistance from the resistance BOX AC : Metal wire one metre long Rh : Rhoeostat E : Cell K : Plug key K' = Jockey A cell of emfE, keyK and rheostat Rh are connected in series with the bridge wire AC. The juncation B of thegalvanometer and the resistance box is connected to thejockey which can slide along wire AC. A suitable resistance 'R' is taken in the resistance box and a curent 'J' is send round the circuit.Withouttouchingthe jockey toany point of AC,note the deflection in the galvanometer. A rheostat is adjustedto get a suitable deflection(e.g.`0.15, 20` divisions) in the galvanometer. Place the jockey at points A and C and see the deflection on the galvanometer. It should be on opposite sides. By touching the jockey to differentpoints of wire AC, find (obtain) the point of contactD for which the galvanometer showsthe same deflection as before i.e, points B and D are equipotential (i.e,the point givesthe same deflection in the galvanometer with orwithout the contact of the jockey.) In this method,the null pointis not obtained. thus, Kelvin's method is a deflectionmethod. The point D is called the balancedpoint. LET, `l_(g)` and `l_(r)` be the distances of point D from ends A and C of wireAC respectively. The resistanceper unit lengthof wire ACis `sigma`. Here alsoG,R and resistances of wire of lengths `l_(g)` and `l_(r)` FORM FOUR armsof a balancedWheatstone's network. `G/R = ("Resistance of wire AD of length" l_(g))/("Resistance of wire CD of length" l_(r))` `= :. G/R = (sigmal_(g))/(sigma l_(r)) = ((l_(g))/(l_(r)))` `:. G = R ((l_(g))/(l_(r)))` `:. G = R ((l_(g))/(100 - l_(g)))` Thus the resistanceof galvanometer G can becalculatedby knowingthe valuesof R and `l_(g)`in the ABOVEEQUATION . |
|