Appendix C 607 



tendency for charge to flow. In many bioelectrical experiments, the 

 physical quantity measured is a potential difference in volts. 



In intermediate courses in electricity, the potential V is defined in 

 terms of the electric field strength E by the equation 



E = -VV 



The electric field strength is the magnitude and direction of the force 

 per unit charge at a given place in space. Symbolically, E is defined by 



E = ^ 



dq 



The value of E determines among other things the point at which an 

 insulator will "break down" passing a spark to ground or the surrounding 

 medium. This discharge is used in spark plugs to cause a spark when 

 the field strength at its points becomes sufficiently great. In dry air, 

 the critical field strength is about 3 x 10 6 v/m. If the spark plug is 

 sufficiently worn away, the separation between the two points becomes 

 too large and, accordingly, the field strength never attains this value. 



If a direct (unidirectional) current flows between two points, then 

 one may compute the ratio R called the resistance, defined by 



-J 



Ohm's law states that R is independent of /. It is approximately 

 true for many substances over large ranges of /. Substances for which 

 R is low are called conductors, those for which R is high are insulators. 



If other forms of energy are reversibly converted to electrical energy 

 when a current flows, the generated potential difference is called an 

 electromotive force (emf). It is often distinguished by the symbol E. 

 Electrochemical cells, electromechanical transducers such as motors and 

 microphones, and junctions between dissimilar metals all give rise to an 

 emf. When the emf causes the current to flow, nonelectrical energy is 

 converted to electrical energy. If the current is caused to flow in the 

 opposite direction, electrical energy is removed by the emf. Some 

 direct current circuits are illustrated in Figure 1 of Chapter 4. 



A term used frequently in discussing biological tissues is capacity. If 

 two conductors are separated by an insulating medium, then the ratio 



L - V 



is called the capacity. A current cannot flow from one conductor to the 

 other through the capacitor. However, while the capacitor is being 



