But in reservoir work such coatings are not to be trusted, and a single horizontal crack might admit sufficient water to cause an uplift.
The spark may be obtained from the secondary of an induction coil, whose terminals are in connexion with the coatings of a Leyden jar.
The positive and negative electrifications of the two coatings of the Leyden jar were therefore to be regarded as the result of a transformation of something called electricity from one coating to the other, by which process a certain measurable quantity became so much less on one side by the same amount by which it became more on the other.
Dr John Bevis of London suggested, in 1746, the use of sheet lead coatings within and without the jar, and subsequently the use of tin foil or silver leaf made closely adherent to the glass.
For charging in parallel a number of jars are collected in a box, and all the outside coatings are connected together metallically and all the inside coatings brought to one common terminal.