6o 



ELEMENTARY LESSONS ON [CHAP. I. 



battery, Fig. 33. As it has a large capacity it will 

 require a large quantity of electricity to charge it fully. 

 When charged it produces very powerful effects ; its 

 spark will pierce glass readily, and every care must be 

 taken to avoid a shock from it passing through the 

 person, as it might be fatal. The " Universal Dis- 

 charger " as employed with the Leyden battery is shown 

 in the figure. 



55. Seat of the charge. Benjamin Franklin 

 discovered that the charges of the 

 Leyden jar really resided on the 

 surface of the glass, not on the 

 metallic coatings. This he proved 

 by means of a jar whose coatings 

 could be removed, Fig. 34. The 

 jar was charged and placed upon 

 an insulating stand. The inner 

 coating was then lifted out, and the 

 glass jar was then taken out of the 

 outer coating. Neither coating 

 was found to be electrified to any 

 extent, but on again putting the jar 

 together it was found to be highly 

 charged. The charges had all the 

 time remained upon the inner and 

 outer surfaces of the glass dielectric. 

 56. Dielectric Strain. Fara- 

 day proved that the medium across 

 which induction takes place really 

 plays an important part in the 

 phenomena. It is now known 

 that all dieletrics across which inductive actions are at 

 work are thereby strained}- Inasmuch as a good 

 vacuum is a good dielectric, it is clear that it is not 



1 In the exact sciences a strain means an alteration of form or volume 

 due to the application of a stress. A stress is the force, pressure, or other 

 agency which produces a strain. 



Fig- 34- 



