WHEATS-TONE'S BRIDGE. 335 



In the apparatus of Siemens, the two coils of the differential 

 galvanometer are removed from each other by a fixed quantity ; 

 they are displaced parallel to each other in respect of the needle, 

 until this comes to zero. An empirical graduation gives the ratio 

 of the intensities of the two currents. 



This ratio is also given by the two rectangular frames of 

 Fleeming Jenkin (854), when the system is turned through an 

 angle such that the needle remains in the meridian. If </> is the 

 angle through which it has been necessary to turn the system 

 from the meridian, we have 



k i k g+r 



942. WHEATSTONE'S BRIDGE. The arrangement known as 

 Wheatstone's Bridge* was first devised by Christie,! and applied 



Fig. 184. 



by him to the measurement of resistances, as long ago as 1833. 

 It is an arrangement of six wires, which may be represented as 

 the four sides and two diagonals of a quadrilaterial. From the 

 original form of Wheatstone's apparatus, in which the wires termi- 

 nated in the four corners of a lozenge, it is sometimes called the 

 parallelogram of resistances. One of the diagonals contains the 

 battery, the other a galvanometer ; the experiment consists in 



* WHEATSTONE. An Account of Several New Instruments and Processes 

 for Determining the Constants of a Voltaic Circuit. The Bakerian Lecture for 

 1843. Phil. Trans., Vol. cxxxm., pp. 303, 327. Scientific Papers, p. 127. 



t CHRISTIE. Experimental Determination of the Law of Magneto-Electric 

 Induction. Phil. Trans, for 



