ELECTRO-PHYSIOLOGY. 577 



resistance the current experiences in passing through the liquid of the 

 cell from one plate to another; and external resistance, or the resist- 

 ance the current meets with in passing through the electrodes and 

 apparatus. Internal resistance is inversely proportional to the size 

 of the cell, and directly proportional to their distance from one 

 another; that is, the larger the plate the less the resistance, and the 

 greater the distance the greater the resistance, the conducting power 

 of the liquid being always the same. External resistance depends 

 on the conductivity of the conductor, which is a constant quantity for 

 each conductor. External resistance is directly proportional to the 

 length of the conductor and inversely proportional to the cross-sec- 

 tion ; that is, the longer the conductor the greater the resistance, and 

 the thicker the (wire) conductor the less the resistance. The thinner 

 the wire the greater is the resistance. 



Batteries may be united together as positive pole to negative 

 pole. Here the voltage is equal to the voltage of a single cell multi- 

 plied by the number of cells. This method of coupling is used in the 

 medical battery for the application of the galvanic or constant cur- 

 rent to man. Another method is to couple abreast or "in multiple 

 arc." Here the positive poles are on one wire and the negative on 

 another wire. Here we have, as a matter of fact, a single cell with 

 plates as many times larger as we have taken cells. The electromotive 

 force is not altered, since the electromotive force of a cell varies with 

 its chemical constituents and not with the size of the cell. Now, the 

 internal resistance of a cell is inversely proportional to the size of the 

 plates, so that by multiplying the size of the plates by the number of 

 cells, say six, then the internal resistance is practically diminished 

 one-sixth. Increased quantity of current is, therefore, obtained. 



The human body opposes to the electric current so great an 

 ernal resistance that the internal resistance of the battery can be 

 srlooked; hence surface extent of the zincs can be neglected. The 

 intensity of the current is determined by the number of the elements 

 and not by their size, hence you couple in series. When, however, 

 you employ electricity to heat the galvano-cautery wire, which is 

 short and of feeble external resistance, you augment the intensity of 

 the current by increasing the surface (size) of the zincs. It is true 



I you do not augment the electromotive force; but as the resistances 

 diminish in proportion to the increase of size of the zinc, the inten- 

 sity of the current increases in proportion to the increase of size of the 

 .cs. You can have an apparatus to heat the cautery wire by coup- 



