158 PHYSIOLOGICAL PHYSICS. [Chap. xv. 



the current to the deep parts and to affect the skin as 

 little as possible, the electrodes are moistened, and the 

 skin also, with warm salt water or acidulated water ; 

 and when it is desired to affect the skin only, dry 

 electrodes are used and the skin is dried and powdered. 



Oalvaiiism may be applied in two chief ways. 

 (1) The one electrode may be placed on some indif- 

 ferent part of the body (the nape of the neck, the pit 

 of the stomach, or held in one hand), while the other 

 electrode is applied to the the part it is desired to 

 influence, one side of the head, over the pneumo- 

 gastric or sympathetic, or to a particular muscle, 

 the two places being distant from one another. 



(2) The two poles may be near to one another, 

 e.g. one at one side of the head and the other at the 

 other side, to influence the brain. 



In the former case the current, entering at the 

 place of the positive pole in a dense stream, spreads 

 itself out in various directions in streams of less 

 density, and is then collected into one to pass out by 

 the negative pole. In such a case the current, being 

 broken up in its passage through the body, will have 

 its principal effect at the two poles, where its density 

 is greatest. In the latter case, the two poles being 

 near one another, the current will pass in one stream, 

 as it were, and therefore with almost undiminished 

 density. 



When a current passes in the opposite direction 

 to the ordinary nerve current it is said to be inverse 

 (up a limb, for instance), when in the same direction 

 it is direct. 



In the next place the current may be sent con- 

 tinuously through the body or part of it, or it may be 

 interrupted by keeping one electrode fixed, and alter- 

 nately lifting and then reapplying the other, so as to 

 make and break the circuit. The interruption may 

 also be effected by a cogged wheel, or some other 



