STIMULI AND THEIR ACTIONS 



407 



electricity in the galvanic element, it is necessary to graduate 

 finely and in any way desired the intensity of the current. For 

 this purpose we must consider somewhat fully the fundamental 

 law which formulates the facts regarding the intensity of elec- 

 tricity. This is Ohm's law; it may be stated as follows: the 

 strength of a current is directly proportional to the electromotive 



E 



force, and inversely proportional to the resistances: 1= 



W 



The electromotive force depends upon the kind and number of the 

 elements. Many elements have only slight electromotive tension, 

 others very high tension ; and if two or more elements be coupled 

 together so that unlike 

 poles are joined with one 

 another, the current is con- 

 siderably stronger than 

 that afforded by a single 

 element. According to 

 Ohm's law, the chief means 

 of strengthening or weak- 

 ening the intensity / of a 

 current consists in increas- 

 ing or diminishing the 

 number of the elements, 

 for thereby the electro- 

 motive force E is increased 

 or diminished. But this 

 graduation by change of 

 the electromotive force is 

 very crude and does not 

 allow T delicate changes to 

 be made. Hence, where 

 finer graduations are re- 

 quired, the second factor 

 upon which, according to 

 Ohm's law, the intensity 

 depends, is employed, 



namely, the resistances W. The resistances are of two kinds : on 

 the one hand, internal resistance, that which exists in the element 

 itself, especially in the liquid, which is a moist and, therefore, a bad 

 conductor of electricity; on the other hand, external resistance, 

 which exists outside the element in the kind, the length, and the 

 diameter of the conductor. The latter especially can be graduated 

 very delicately. 



Metals are good conductors, and for this reason metallic wires, and 

 best copper wires, are always selected as conductors outside the ele- 

 ment. Their resistance is less, the shorter the conduction and the 

 greater their cross-section. A very ready means of increasing the 



FIG. 191. A non-polarisable electrode. A glass tube 

 closed by a stopper of clay and filled with a concen- 

 trated solution of zinc sulphate is held in the movable 

 stand. A moist camel's-hair brush sticks into the 

 clay stopper, and a zinc rod, to which the wire is 

 carried, dips into the solution. The nerve of the pre- 

 paration is laid over the brushes of two such elec- 

 trodes. 



