PHYSIOLOGICAL PHYSICS. 



[Chap. VI. 



APf 



side of the key carry a wire to the apparatus to which 

 the current is being conveyed, represented by APP. 



From the apparatus take a 

 wire to 4 011 the opposite side 

 of the key, and from 3 on 

 that side take a wire back to E 

 to the negative pole N. Lower 

 the bridge so as to " close the 

 key," and now follow the 

 direction of the current. It 

 passes from P to A ; but as 

 soon as A is reached two path- 

 ways are open, one by the wire 

 from 2 round the apparatus, 



Fig. 31. Friction Key in ,-, -, u i 



Short Circuit. then on to B, and so on back 



to E, the other from A, straight 



across the bridge to B, so gaining 3, and passing to E. 

 The first is a long route presenting considerable resis- 

 tance ; the second is a short circuit, and, since the 

 bridge is of large section, presents no resistance to 

 speak of. When, therefore, the key is closed, all the 

 current will pass straight across the bridge back to 

 the battery, and none will go the long route, owing to 

 the great difference of resistances. The battery cur- 

 rent is then said to be short-circuited, arid the key is 

 interposed in SHORT CIRCUIT. * Another term for 

 short circuit is accessory circuit. 



When, however, the bridge is raised, the key is 

 opened, and in that case the current has no option, but 

 must go the long route, the short circuit being inter- 

 rupted. With a key in short circuit, therefore, closing 

 the key means interrupting the current in the apparatus^ 

 and opening the key means sending on the current to 

 the apparatus. The same key may be used in a simple 



* For the sake of those who read German and who might have difficulty 

 in fiuding the meaning of the word, it maybe noted that nebenscldiessung 

 is used as we use short circuit. 



