178 Normal Elements for Electrometric Measurements. 



The dry Daniell elements have therefore this in common 

 with Latimer-Clark's element, that they can be prepared once 

 for all ; but they have farther this advantage, that they are 

 scarcely at all influenced by temperature ; and that an acci- 

 dental closing of the circuit produces only an insignificant 

 decrease even in elements of small size (about 1 per cent.), 

 which is very soon recovered. If the electromotive force of a 

 Daniell element put together with copper, zinc, concentrated 

 copper-sulphate solution, and concentrated zinc-sulphate solu- 

 tion be taken as 1*059 volt, then the mean electromotive 

 force of a dry Daniell element = 1*056. It must, however, 

 be remembered that the value 1*059 is based upon the assump- 

 tion that the force of a Latimer-Clark element =1*457 volt, 

 which estimate is only to be considered as a preliminary one. 



The resistance of element I. was 14,600 ohms, of element II. 

 13,500 ohms. The strength of current passing through a 

 short-circuited element is therefore 0*000072 ampere for ele- 

 ment II., and 0*000078 ampere for element III. ; i. e. in the 

 first 008 milligrm. copper would be deposited in the hour, and 

 with the latter 0*09 millgrm. copper. 



The dry Daniell elements are to be recommended for another 

 purpose, viz. for charging a quadrant-galvanometer. Zam- 

 boni's pile has not proved successful for this purpose ; it acts 

 excellently for a time, but then the potential difference of its 

 poles changes suddenly, especially with irregular changes of 

 temperature. Apparently the deposition of moisture on the 

 inside of the glass tube containing the pile produces a partial 

 short-circuiting of the pile. With a water-battery these 

 sudden changes do not occur, but the difference of potentials 

 decreases gradually, if slowly. Errors in measurement cannot 

 be caused thus; but the deflections of the electrometer become 

 smaller and smaller, and, apart from the necessity of renew- 

 ing the evaporated water from time to time, the whole battery 

 every now and then has to be disconnected and cleaned from 

 deposits of oxide and carbonate. I found the electromotive 

 force of a newly prepared zinc and copper element filled with 

 spring-water to be 0*992 volt. After twelve hours, during 

 which the element remained open, it had sunk to 0*934 volt. 

 Three of the elements forming my water-battery, which has 

 stood now more than a year, were examined. They showed 

 the following differences of potential : — 



0*838 0-678 0*724 volt Mean 0*743 volt. 



The dry Daniell elements may conveniently be combined 



