3 6 4 



COMPARISON OF RESISTANCES. 



966. WIRE BRIDGE. This modification of Wheatstone's bridge 

 is due to Kirchhoff.* It is especially suitable for measuring small 

 resistances and for comparing standards. 



In the ordinary parallelogram (Fig. 190), one of the summits C 

 is replaced by a straight wire A'B', along which a movable contact 

 may be displaced. Instead of varying one of the resistances, the 

 balance is made by a suitable displacement of the point C along 

 the wire. 



If / be the length A'B' of the wire supposed to be homogeneous 

 and regular, x the distance A'C, then expressing the resistances 



C B 



Fig. 190. 



a and b of the sides AA' and BB' in units of length of the wire, 

 the condition of equilibrium will be 



a a + x 

 ~b' = b + l-x' 



The wire is usually about a metre in length and i '5 mm. to 2 mm. 

 in diameter. It is of brass or of argentan, or better of platinum- 

 iridium (85 platinum and 15 iridium). This alloy has the advantage 

 of not being oxidizable and of not amalgamating. 



The wire forms one side of an elongated rectangle (Fig. 191), 

 and the three other sides are formed of broad copper strips, the 

 resistance of which may be neglected. These strips have breaks 

 at P, Q, P' and Q', which may be closed, either by thick copper 

 strips or by resistances 0, , a', b'. The two points A and B are con- 

 nected by the battery, the galvanometer wire is attached at D and 

 to the movable contact C. By means of a divided rule the position 

 of this contact may be determined. 



* KIRCHHOFF. Fogg. Ann., Vol. c., p. 177. 1857. 



