246 STUDIES IN ELECTRO-PHYSIOLOGY: 



Furthermore, the Farad is a unit as, for instance, a 

 gallon of the capacity of a cistern into which the water 

 may be caused to flow from E, and in which the quantity 

 of one coulomb produces a pressure of one volt, by creating, 

 as it were, another head of water at a lower level. 



For a circuit to be established it is necessary in the case 

 of electricity for there to be a return, either by another wire 

 or by the earth ; there must be a " loop." Similarly no 

 water will flow from the cistern unless it has access to air, 

 nor will any water issue from a pipe unless and until the 

 tap is opened to air. 



The resistance of a metallic conductor is directly pro- 

 portionate to its length, is in the inverse ratio to its 

 sectional area, and is expressed by R. There are, however, 

 resistances (r) other than that of the conductor or conduc- 

 tors to be taken into account, and the principal of these 

 (outside the galvanometer and electrodes) is the internal 

 resistance of the generating cell or cells. This varies not 

 only with the surface area of the plates but in a galvanic 

 cell with the chemical composition of the exciting fluid. 



Briefly summed up, the E.M.F. is proportional to the 

 current when the resistance is constant, the E.M.F. is 

 proportional to the resistance when the current is constant, 

 and the E.M.F. is proportional to the product of current 

 strength and resistance when both vary. 



The resistance of metals increases with rise of tempera- 

 ture, while that of liquids and dielectrics decreases more or 

 less rapidly. 



When there are two conductors of different resistance 

 joining two points, the current in either branch is inversely 

 as the resistance of that branch. 



In reviewing the galvanometric deflections exhibited in 

 normal health by the human body we must bear in mind 

 certain facts of primary importance. The conductors 

 (nerves) and condensers (certain cells) are composed of 



