649 



[Barker. 



who had obtained from them a diatonic scale of an octave 

 and a quarter in extent. 



The Librarian reported the completion of his MS. con- 

 densed copy of the early records of the Proceedings of the 

 Society from 1744 to 1837. The subject of printing the 

 same was referred to the Committee of Five (Phillips, Horn, 

 Lewis, Brinton and Law) appointed December 16, 1881. 



And the meeting was adjourned. 



On the Meamrement of Electromotwe Force. By George F. Barker. 

 {Read before the American Philosophical Society, January 19, 1SS3.) 



The term electromotive force is applied to that force which tends to set 

 electricity in motion. It appears to have been used first by Ohm, who in 

 1827 gave precision to the study of electric currents by formulating his 

 well known law: — The strength of an electric current is directly propor- 

 tional to the sum of the electromotive forces and inversely proportional 

 to the sum of the resistances in the circuit. 



The measurement of electromotive force may be absolute or relative ; 

 absolute when it is determined directly, relative when its value is obtain- 

 ed by comparison, the ratio of an unknown to a known electromotive 

 force being the object of the measurement. In both measurements, the final 

 standard of electromotive force is an absolute unit, based upon the funda- 

 mental units of mass, length and time ; since these are respectively 

 the centimeter, the gram and the second, absolute units are often 

 called C. G. S. units. In electrostatics, electromotive force and difference 

 of potential are synonymous, the same unit being used for both. The 

 unit difierence of potential exists between two points, when to carry a unit 

 of positive electricity from one to the other, requires the expenditure of 

 a unit of work ; or in the C. G. S. system, of an erg. Now a unit of 

 work, i. 6., an erg, is done when a unit of force, i. e., a dyne, overcomes 

 resistance through an unit of distance, i. e., a centimeter. And a unit of 

 force, i. e., a dyne, is that force which produces a unit of velocity in a unit 

 of time ; i. e., produces an increase of velocity of one centimeter in one 

 second. Since in this latitude, gravity produces a velocity of about 980 

 centimeters per second, the force of a dyne corresponds to the attractive 

 force which the earth exerts upon the l-980th part of a gram. To raise 

 one gram therefore to the height of one centimeter requires the expendi- 

 ture of 980 ergs of work. Obviously then if two electrified bodies at unit 

 distance attract or repel each other with a force equivalent to that which 



