THE SMITHSONIAN INSTITUTION. 371 



strength of the currents will deflect the needle. But since the sum 

 of all the resistances is not great here, a very minute change in the 

 resistance inserted hetween e and/ will cause a sensible change in the 

 strength of the current, and therefore a sensible deflection of the needle. 



Now, to obtain by this contrivance the resistance of a wire expressed 

 in turns of the rheostat, the following method can be adopted : In- 

 sert between e and/ a few of the turns of the rheostat, and between 

 g and h a wire, whose resistance is nearly equal to that of the inserted 

 part of the rheostat on the other side, and adjust everything so that 

 the needle may come to rest at 0*. Now, inserting between g and A, 

 besides the wire already there, the wire whose resistance is to be de- 

 termined, there must be inserted on the other side a series of n turns 

 of the rheostat to bring the needle back again to 0. This number n 

 of revolutions of the rheostat wire is the measure of the resistance of 

 the wire in question. 



Wheatstone has constructed other instruments besides this for the 

 same object ; but the description of this, the simplest one, will suffice. 



SECTION THIRD. 

 RESISTANCE OF METALS AND LIQUIDS, GALVANIC POLARIZATION AND PASSIVITY. 



§ 28. In order to compute by Ohm's formula the strength of current 

 in a given case, it is not sufficient to know merely the constants of the 

 electro-motor — we must also know the resistance of the solid con- 

 ductors which are inserted in the closing circuit ; and in case the 

 current has to traverse a decomposing cell, besides the resistance of 

 the liquids, we must also know the electro-motive opposing force ap- 

 pearing at the electrodes, or what is called the galvanic polarization. 

 The conduction of the current, it is well known, depends upon the 

 dimensions of the body, and also on its specific conductive capacity, 

 which we shall now consider. 



§ 29. Resistance of metals. — Bufi" has determined the resistance of 

 a few of the metals by Wheatstone' s method, as follows (Jahres- 

 bericht von Liebig und Kopp fiir 184*7 and 1848, s. 286:) 



Silver 0.954 



Copper, (chemically pure,) 1.000 



Copper of commerce, first quality 1.170 



Do second quality 1.507 



German silver 11 . 833 



He has taken the resistance of silver as unity ; but since all re- 

 sistances have been compared here with copper, I have reduced the 

 data of Buff to this metal. 



To distinguish the absolute value of resistance of a wire from these 

 proportional numbers, I propose to call them the specific resistance to 

 conduction. The specific resistance to conduction of a metal is the 



■-" To facilitate such an arrangement Wheatstone has introduced a special contrivance 

 info his instrument. The knob d rests firmly upon a piece of brass. At the other end 

 of this strip of brass anotlier piece n turns about a pin, its free end resting on the wire. 

 When n lies on d it has no effect, but the further it is turned from d towards g the more 

 will the resistance on the course d g\>c reduced. If necessary the movable piece of brass 

 n can also be brought to the other side of d. 



