OF ARTS AND SCIENCES. 113 



if the primary circuit of the Ruhrakorf coil is broken by a pendulum. 

 "With the ordinary automatic break, such as is commonly employed 

 upon induction coils, the failure of a single break is unimportant. If, 

 however, a single break fails when a pendulum break is employed, the 

 record of the experiment is an imperfect one. An excess of battery 

 power and a large battery are therefore necessary. A metallic break- 

 piece, also, was found to be more inconstant for our purposes than a 

 mercury break. 



It was found that a certain simplicity of contrivance was necessary 

 in the method of discharging the air condenser through the inductance 

 circuit. No arm connected with the revolving mirror could be trusted 

 to break or make an electrical circuit by throwing in or out any form 

 of switch. The great speed at which it revolved broke all arrange- 

 ments which were tried. By placing a short stiff brush of minute 

 size upon the end of the flying terminal connected with the revolving 

 mirror, and allowing this brush to rub against a brass plate set in an 

 ebonite circle of 41 cm. in diameter, constancy of action was secured. 



In order to obtam the same difference of potential at each run, ex- 

 periments were first made with various forms of unit jars and pith 

 ball electrometers. These devices were speedily given up in favor of 

 a simple balance electrometer. One of the pans of a delicate balance 

 wjis replaced by a metallic disk. A similar disk, which was stationary, 

 was placed immediately below the movable one. By properly weighting 

 the remainmg balance pan, great delicacy and range of indication were 

 obtained. This apparatus constitutes, in fact, an absolute electrome- 

 ter. A suitable guard ring can be placed around the movable disk. 



When the air condenser had been charged to a definite potential, 

 the movable disk of the electrometer closed an electrical circuit in 

 which was included an electrical bell. The observer stationed at the 

 chronograph, at the instant he heard the bell, drew the carriage con- 

 nected with the stylus along the guides which kept the stylus on the 

 blackened cylinder. 



Calling L the coefficient of self-induction, we have 



■^ = 2 1og^, + 1,* 

 / ° a- ■ 



in which / is the length of conductors contained between two parallel 



planes; 6, the distance apart of the conductors; o, the radius of wires. 



In our case the effect of the ends was found to be inappreciable. 



The induction due to the ends can be calculated by the repeated em- 



* Mnxwell, § 085, Vol. II. 



VOL. XXV. (n. S. XVII.) 8 



