496 



Contact ivith Dielectrics. 



is —9. And it is seen, taking the readings at the 6th ? 16th, 

 26th, 36th, and 46th min., that we obtain the sequence —10*5, 

 4-7*5, —9, +7*5, —9; a further indication that with mercury 

 electrodes the apparatus conditions are trustworthy. These 

 earth-readings were taken without a shunt, so that they are 

 actual records of direct deflexions. 



Retentive Force. — It seemed desirable to obtain positive 

 evidence as to the magnitude of the retentive force between 

 the tinfoil and the press-spahn, and to determine whether it 

 was proportional to the voltage. For this purpose, a flat 

 circular disk of gutta-percha, of the same size as the electrodes 

 used in the foregoing experiments, was perforated over about 

 half its surface with holes 1 cm. in diameter. A smooth 

 sheet of thin tinfoil, not perforated, was attached to one side 

 of this disk by a little chloroform. This arrangement allowed 

 a certain amount of freedom of movement to the tinfoil, so 

 that it could conform to the surface of a sheet of press-spahn 

 when placed in contact with it. The disk was then suspended 

 above a sheet of press-spahn, in a horizontal plane, from the 

 beam of a balance, and its weight was counterpoised.. Beneath 

 the sheet of press-spahn was a second sheet of tinfoil con- 

 nected to earth, so that the combination formed a condenser, 

 the upper electrode of which was thus in balanced contact 

 w T ith the dielectric. The weight necessary to counterpoise 

 the disk was first determined with no voltage between tho 

 electrodes, and the weight necessary to relieve it from con- 

 tact was then measured at successive voltages. Check read- 

 ings of the weight, taken between the various applications of 

 voltage, never exhibited a difference greater than 1 centi- 

 gramme for the normal (no- voltage) counterpoise. If there 

 was residual coherence, it was therefore very small. The 

 readings in a characteristic case were : — 



Table IX. 



Volts. 

 



Counterpoise. 



Retentive force, 



Eatio of 



Ratio of 



Total grammes. 



grammes. 



Retentive forces. 



Volts. 

 



22-04 











750 



72-00 



52 



104 



5 







22 04 















600 



50-00 



28 



56 



4 



450 



25-00 



3 



6 



3 



300 



23-00 



1 



2 



2 



150 



22-50 



0-5 



1 



1 



300 



2300 



1 



2 



2 



450 



25-00 



3 



6 



3 



600 



49-00 



27 



54 



4 



750 



66-00 



44 



88 



5 



