.s/A' \\-JLLIAM SIEMENS, F.R.S. 



437 



nuin wire, the reading of the galvanometer, divided by the main 

 current as determined by the electro-dynamometer, is proportional 

 to the resistance of the wire. Hence the constant of the instru- 

 ment and the resistance of the galvanometer being known, the 

 resistance of the platinum wire could be calculated, as the current 

 was varied by altering the number of cells composing the 

 battery. 



The measurements were made in all cases when equilibrium had 

 been established between the radiation and the energy of the 

 current, as evinced by the constancy of the readings of the electro- 

 dynamometer and galvanometer. 



Having made a rough preliminary series of experiments to test 

 the suitability of the method and apparatus, with satisfactory 

 results, on April 17th I made a second series, the results of 

 which are recorded in Table I. Column I gives the current in 

 amperes passing through the wire ; column II the difference of 

 potential in volts between the terminals as deduced from the 

 readings of the galvanometer ; column III the rate at which the 

 energy of the current was converted into radiant energy, repre- 

 sented by the product of the electromotive force and current, and 

 therefore measured in volt-amperes or watts ; column IV the 

 resistance of the wire, being the ratio of the electromotive force 

 to the current ; column V the corresponding temperature of the 

 wire in degrees Centigrade. Finally, column VI describes the 

 condition of the wire as apparent to the eye. 



TABLE I. 



LENGTH OP WIRE 102 CENTIMS. DIAMETER 0'76 MILLIM. 

 TEMPERATURE OF ROOM 65 FAHR. 



