MUSCLE 535 



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 



AB 



. .. . a CD BC CD AB BC 



Similarly : rx!) ; .-. ^B = AD' r AD = CD' 



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

 number of coils of wire of different resistances, is used (Fig. 148). 

 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 



FIG. 149. SCHEME OF WIEDEMANN'S GALVANOMETER (WITH TELESCOPE 



READING). 



T, telescope ; S, scale ; M, mirror ; m, ring magnet suspended between the two 

 galvanometer coils G, the distance of which from m can be varied ; F, fibre suspending 

 mirror and magnet. 



of T : 10, i : ioo, 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 

 ray of light is allowed to fall on the mirror, from which it is reflected 

 on to a scale ; and the rotation of the mirror is magnified and 

 measured by the excursion of the spot of light on the scale. In 



