408 



GENERAL PHYSIOLOGy 



resistance and thereby diminishing the intensity of the current by 

 any desired amount is that of lengthening the conducting wire and 

 using wires that have a small cross-section. 



Upon these facts is based a principle which comes into general 

 use in apparatus that is employed for graduating the intensity of 

 the current, viz., the principle of the accessory or short circuit. If, 

 e.g., a circuit from an element E (Fig. 192, 1) extend through 

 copper wires to a preparation N, a galvanic current of a definite 

 intensity, which can easily be measured, flows through the 

 preparation, although the latter as a moist conductor affords 

 considerable resistance. But, if into this circuit a short circuit be 

 introduced by joining two opposite points of the metallic 

 conductor by means of a cross-wire, a small circuit is made to 



u E 



Fio. 192. 



-Scheme of the short circuit. /, A simple circuit ; //, long circuit with short circuit. 

 E, element; iV, nerve-muscle preparation ; A, B, short circuit. 



branch off from the large one ; in the former the resistance is con- 

 siderably less than in the latter, since its conductors are metallic 

 and shorter than those in the latter. The result is, as Ohm's law 

 directly teaches, that in the long circuit a current of slight inten- 

 sity passes, which is so feeble that under certain circum- 

 stances it has no effect whatever upon the preparation, while in 

 the short circuit there is a current of considerable intensity. 

 Hence in the long circuit, in which the preparation is, there are 

 two extremes of intensity: with the short circuit broken, a 

 considerable current, and with the short circuit closed, a very 

 slight current. From the latter extreme to the former the 

 intensity can be graduated very delicately by successively increas- 

 ing the resistance in the accessory circuit, until it becomes so 

 great that the circuit hardly conducts at all. Then nearly the 

 whole current goes through the large circuit and the preparation. 



