542 ohm's law, strength and density of galvanic currents. 



other bodies, the first substance is negatively, the last positively, electrified. ' This series is : 

 -carbon, platinum, gold, silver, copper, iron, tin, lead, ziuc + . 



The amount of the electro-motive force produced by the contact of two of these bodies is 

 greater, the further the bodies are apart in the series. The contact of the bodies may take place 

 at one or more points. If several of the bodies of this series be arranged in a pile, the electrical 

 tension thereby produced is just as great as if the two extreme bodies were brought into contact, 

 the intermediate ones being left out. 



2. The nature of the two electricities is readily determined by placing one of the bodies of the 

 series in contact iritk a fluid. If zinc be placed in pure or acidulated water, the zinc is + 

 (positive) and the water - (negative). If copper be taken instead of zinc, the copper is + but 

 the fluid - . Experiment shows that those metals, in contact with fluid, are negatively electri- 

 fied most strongly which are most acted on chemically by the fluid in which they are placed. 

 Each such combination affords a constant difference of tension or potential. The tension [or 

 power of overcoming resistance] of the amount of electricity obtained from both bodies depends 

 upon the size of the surfaces in contact. The fluids, e.g., the solutions of acids, alkalies, or salts 

 are called exciters of electricity of the second class. They do not form among themselves a 

 definite series with different tensions. When placed in these fluids, the metals lying next the 

 + end of the above series, especially zinc, are most strongly electrified negatively, and to a less 

 -xtent those lying nearer the - end of the series. 



3. Galvanic Battery. If two different exciters of the first class be placed in fluid, without 

 the bodies coming into contact, e.g., zinc and copper, the projecting end of the (negative) zinc 

 shows free negative electricity, while the free end of the (positive) copper shows free positive 

 electricity. Such a combination of two electro-motors of the first class with an electro-motor 

 of the second class is called a galvanic battery. As long as the two metals in this fluid are kept 

 separate, the circuit is said to be broken or open, but as soon as the free projecting ends of the 

 metals are connected outside the fluid, e.g., by a copper wire, the circuit or current is made 

 or closed, and a galvanic or constant current of electricity is obtained. The galvanic current 

 lias resistance to encounter in its course, which is called "conduction resistance" (W). It is 

 directly proportional (1) to the length (I) of the circuit; (2) and with the same length of 

 circuit, inversely as the section (q) of the same ; and (3) it also depends on the molecular 

 properties of the conducting material {specific conduction resistance's), so that the conduction 

 resistance, W = (s. I) : q. The resistance to conduction increases with the increase of the 

 temperature of the metals, but diminishes under similar conditions with fluids. 



Ohm's Law. The strength of a galvanic current (S), or the amount of electricity passing 

 through the closed circuit, is proportional to the electro-motive force (E) or the electrical 

 tension, but inversely proportional to the total resistance to conduction (L) 



So that S = E : L (Ohm's Law, 1827). 



The total resistance to conduction, however, in a closed circuit is composed of (1) the resist- 

 ance outside the battery (" extraordinary resistance ") ; and (2) the resistance within the battery 

 itself (" essential resistance "). The specific resistance to conduction is very variable in different 

 substances : it is relatively small in metals (e.g., for copper =1, iron = 6 '4, German silver = 12), 

 but very great in fluids {e.g., for a concentrated solution of common salt 6,515,000, for a con- 

 centrated solution of copper sulphate 10,963,600). 



Conduction in Animal Tissues. It is also very great in animal tissues, almost a million 

 times greater than in metals. When a constant current is applied to the skin so as to traverse 

 the body, the resistance diminishes because of the conduction of water in the epidermis under 

 the action of the constant current ( 290), and the congestion of the cutaneous blood-vessels in 

 consequence of the stimulation. But the resistance varies in different parts of the skin, the 

 least being in the palm of the hand and sole of the foot. The chief seat of the resistance is the 

 epidermis, for after its removal by means of a blister, the resistance is greatly diminished. Dead 

 tissue, as a rule, is a worse conductor than living tissues (Jolly). When the current is passed 

 transversely to the direction of the fibres of a muscle, the resistance is nearly nine times as great 

 as when the current passes in the direction of the fibres a condition which disappears in rigor 

 mortis (Herniann). In nerves, the resistance longitudinally is two and a half million times 

 greater than in mercury, transversely about twelve million times greater (Hermann). Tetanus 

 and rigor mortis diminish the resistance in muscle (du Bois-lleymond). 



Deductions. It follows from Ohm's law that I. If there is very great resistance to the 

 current outside the battery [i.e., between the electrodes], as is the case when a nerve or a muscle 

 lies on the electrodes, the strength of the current can only be increased by increasing the 

 number of the electro-motive elements. II. When, however, the extraordinary resistance is 

 very small compared with that within the battery itself, the strength of the current cannot be 

 increased by increasing the number of the elements, but only by increasing the surfaces of the 

 plates in the battery. 



Strength and Density. We must carefully distinguish the strength (intensity) of the current 

 from its density. As the same amount of electricity always flows through any given transverse 

 section of the circuit, then, if the size of the transverse section of the circuit varies, the elec- 



