492 Dr. C. H. Draper on the 



The readings taken with the liquid boiling were, as might be 

 expected, somewhat irregular, but in all the potential value 

 for the same current was considerably lower than at 80°. 



It is evident, on comparing the values of E and tan 6 at the 

 different temperatures, that for any given value of the current 

 the potential-difference diminishes with rise of temperature. 

 It was then decided to reduce the results to absolute measure, 

 so that something more than comparative values might be 

 indicated. 



Reduction to Absolute Measure. (1) The Current. — For 

 this it was necessary to find the galvanometer-constant and 

 the horizontal intensity of the earth's magnetism for the 

 position of the galvanometer. 



Galvanometer-constant. — The single ring had a mean dia- 

 meter of 43*21 centim., and its calculated constant was thus 

 •2908 ; as determined by the quantity of hydrogen liberated 

 in the electrolysis of water it was *2902. The mean of these, 

 •2905, was taken. 



Determination of H. — The value of the horizontal com- 

 ponent of the earth's magnetism for the position of the 

 galvanometer was determined by means of a Kew unifilar 

 magnetometer to be '1830. 



(2) The Electromotive Force. — To reduce the electrometer- 

 readings to absolute units the E.M.F. of the standard cell 

 had to be ascertained, since the E.M.F. of a Daniell cell has 

 been variously given from *978 volt (Latimer Clark) to 1*137 

 volt (Kohlrauschj, depending on the strength of the solutions 

 employed. 



Determination of the E.M.F. of the Standard Cell.— The 

 following method was adopted to find the E.M.F. The sen- 

 sitive galvanometer employed in PoggendorfFs method was 

 replaced by a condenser and the galvanometer was placed as 

 a shunt to the condenser. 



The accompanying figure shows the connexions. B a 

 battery of two Daniell cells of the gravitation-pattern is 

 connected through C K, a comnmtating-key, with a circuit 

 containing S G, a standard tangent-galvanometer, and M N, 

 a resistance-box containing 12,000 ohms. B 2 , the standard 

 cell, has one terminal joined to P, a movable plug in the 

 resistance-box, and the other terminal to one coating of the 

 condenser, C, of which the other coating is joined to Q, another 

 movable plug. R G is a delicate reflecting-galvanometer 

 joined, through a contact-key, K, with the condenser C. A 

 steady current was allowed to flow round the circuit BMN, 

 and the resistance P Q was so adjusted, without altering the 

 whole resistance M N. that on depressing the key K there 



