CHAPTER VI. 



THE ELECTROMETRIC MEASUREMENT OF THE VOLTAIC POTENTIAL DIFFER. 



ENCE BETWEEN THE TWO CONDUCTORS OF A CONDENSER, 



SEPARATED BY AN IONIZED MEDIUM. 



68. Introductory. The difficulties encountered in the preceding paper 

 (Chapter V, section 57), were made the subject of direct investigation 

 by replacing the fog chamber with a metallic cylindrical condenser, the 

 core of which was an aluminum tube 50 cm. long and 0.63 cm. in diameter, 

 the shell a brass tube 50 cm. long and 2.1 cm. in diameter, coaxial with 

 the former. Sealed radium tubelets could be placed within the aluminum 

 tube or withdrawn from it. Moreover, either the outer coat or the core 

 of the condenser could be joined in turn with the Dolezalek electrometer, 

 the other being put to earth. The conducting system now appears as 

 shown in fig. 26, C being the outer coat or brass shell, A the aluminum 

 core, and r the radium tubes in the cylindrical core. Conductors are 

 earthed at e. BB show the metallic connections with the auxiliary con- 

 densers C' C" , E is one of the insulated quadrants of the electrometer 

 with the highly charged needle N, E being virtually also a condenser. 



A Clark standard cell 5 may be inserted for standardization, but it is 

 otherwise withdrawn. 







J 



e/ 



V \ 



\*_b 



1 I 



C' 







FIG. 26. Disposition of cylindrical condenser C, auxiliary condensers 

 C' C", key to earth K, electrometer E, standard cell S. All earthed at e 

 Diagram. 



Direct experiment showed the self-charging tendencies to come appar- 

 ently from the highly charged needle N, or at least to arise in the electro- 

 meter E. Positive ions are lodged into the conductor EBB A for a positive 

 needle, negative ions for a negative needle. In addition to this, how- 

 ever, there is a voltaic difference, aluminum-brass, at AC when radium is 

 in place and the medium therefore highly ionized. The latter potentials 

 are usually negligible. These are the chief electromotive forces, the first 



76 



