= 20; 



20 ^ 



5 E — e ~ 6 ' 



THE SMITHSONIAN INSTITUTION. 381 



inches. Making the electro-motive opposing force equal e, we have in 

 the first case 



5 E — e 



, = 6 ; 



in the second, 

 therefore, 



hence, 



c = 2 857E. 



Thus the experiment proved not merely the existence of the electro- 

 motive opposing force, hut also determined its magnitude. 



§ 33. Iiesisfance of liquids to conduction. — To determine the proper 

 resistance of liquids we must take the influence o^ galvanic polariza- 

 tion into consideration ; ignorance or disregard of this was the reason 

 why all former experiments for determining the specific resistance of 

 liquids yielded entirely contradictory results. 



Lenz first sought to determine the specific resistant e of a solution 

 of sulphate of copper, free from the influence of polarization, and 

 found the value 



6857 500 ; 

 that is, a solution of sulphate of copper, in the form of a liquid pile, 

 terminated hy metal plates at hoth ends, heing inserted in the closing 

 arc of a hattery, opposed to the galvanic current a resistance 6857 500 

 times greater than a copper rod of the same dimensions, (Pog. Ann. 

 XLIV, 349.) 



Wheatstone proposed an excellent method for determining the re- 

 sistance of liquids independent of polarization. A glass tuhe two 

 ^. „, inches lonq;, and ahout one-half 



Fig. 24. . • . 



inch in interior diameter, (Fig. 

 24) has one-fourth of its circum- 

 ference ground away, leaving a 

 large part of its length open 

 above ; at one end of the tuoe a 

 metal stopper is fastened, termi- 

 nating in a platinum plate; at 

 the other end a movable piston, ending also in a platinum plate, can 

 be brought within one-fourth of an inch of the fixed plate, and re- 

 moved from i. to the distance of five-fourths of an inch. 



To determine the resistance of a liquid, this measuring tube is in- 

 serted with the galvanometer and rheostat, in the closing arc of a 

 constant oattery of about three cu[)S. When the two platinum plates 

 of the tube are one-fourth of an inch apart, the interval is filled with 

 the liquid whose resistance is to be measured, and then by means of 

 the rheostat the deflection of the needle of the multiplier is brought 

 to a given point. The piston is now drawn back one inch, and the 

 interval filled again with the liquid; of course the needle has receded, 



