84 



A make contraction starts from the kathode, a break con- 

 traction from the anode. 



This experiment also shows that the current passes in 

 opposite directions in make and break. 



315. The Break Extea-Cueeent. When a galvanic 

 current traversing the primary coil of an induction machine 

 is made or broken, each turn of the wire exerts an inductive 

 influence on the others. When the current is made the direc- 

 tion of the extra current is against, or in an opposite direfction 

 to that from the battery, but at break it is in the same direc- 

 tion as the battery current.- 



Fig. t9 

 Fig. 19. The Break-Extra Current. 



Apparatus : Battery, two keys, wires, primary coil of 

 induction machine. Arrange the apparatus according to the 

 diagram. Fig. 19. Both keys and the coil are in the primary 

 circuit, the keys being so arranged that either the primary 

 coil, P, or the electrodes attached to key 2 can be short cir- 

 cuited. Test either by electrodes applied to the tongue or by 

 means of a nerve muscle preparation. Close key 1, thus 

 short-circuiting the coil. Open and close key 2. There is 

 very little effect. Open key 1, the current passes continuously 

 through the primary coil. Open key 2 ; a marked sensation 

 is felt, due to the break-extra current. 



316. Rheocoed. The rheocord consists of a brass or 

 German silver wire (in this case 20 meters in length), placed 

 along a square board, with its ends connected with binding 

 posts. On the wire is a " slider " which can be pushed along 

 the wire as desired. On account of the difference in potential 



