MUSCLE 617 



weight of a packet of tea by comparing it with known weights. The 

 Wheatstone's bridge method of measuring resistance depends on the 

 fact that if four resistances, AB, AD, BC, CD, are connected, as in 

 Fig. 204, with each other, and with a galvanometer, G and a 

 battery F, no current will flow through the galvanometer when 

 AB_BC 

 AD "CD" 



For when no current passes through the galvanometer, B and D 

 are at the same potential. Let the fall of potential from C to B or 

 from C to D be ; then, since the total fall of potential from C to A 

 must be the same along either of the paths CBA or CDA, the fall 



FIG. 204. WHEAT- FIG. 205. DIAGRAM OF RESISTANCE 



STONE'S BRIDGE. Box. 



from B to A must be equal to that from D to A. Call this ft. Now, 

 the fall of potential which takes place in any given portion of a 

 circuit is to the whole fall of potential in the circuit as the resistance 

 of the given portion is to the whole resistance. That is, 



a BC 



BC + AB' 



AB 

 BC + AET 



c . .. , CD BC CD AB EC 



Similarly: ^ ^ ; . . m =j&, or Xo = c> . 



In making the measurement, a resistance box, containing a large 

 number of coils of wire of different resistances, is used (Fig. 205). 

 The resistances corresponding to AB and AD, called the arms of the 

 bridge, may be made equal, or may stand to each other in a ratio 

 of i : 10, i : 100, etc. Then, the unknown resistance being CD, 

 BC is adjusted by taking plugs out of the box till, on closing the 

 current, there is either no deflection, or the deflection is as small as 

 it is possible to make it with the given arrangement. 



Galvanometer. A galvanometer is an instrument used to detect a 

 current, to determine its direction, and to measure its intensity. 

 Since, by Ohm's law, electromotive force, resistance, and current 

 strength are connected together, any one of them may be measured 

 by the galvanometer. A galvanometer of the kind ordinarily used 

 in physiology consists essentially of a small magnet suspended in the 

 axis of a coil of wire, and free to rotate under the influence of a 

 current passing through the coil. The most sensitive instruments 

 possess a small mirror, to which the magnet is rigidly attached. A 



