MAGNETISM AND ELECTRICITY. 153 
c. If one side of the Franklin pane be touched by one ball, and the other 
ball be made to touch the other side, there will be an instantaneous 
exchange of electricities along the brass arms. The glass handles serve as 
a measure of precaution in preventing any shock, either direct or by 
induction, to the individual performing the experiment. 
The Leyden jar (pl. 20, figs. 35, 36) acts on the same principle with the 
Tranklin piate. It consists of a cylindrical glass vessel, open above, and 
coated on the bottom and sides of both surfaces with tin foil, to within a 
short distance of the top. Into the top a wooden cover, gg’, is made to fit 
accurately. Through the cover passes a brass rod, pointed above with a 
ball, 6, to screw on over.this point ; from the lower end hangs a fine chain, 
the extremity of which rests on the tin foil coating on the inside of the jar. 
The wooden cover and the uncovered sides should be coated with lac or 
sealing-wax varnish. Instead of the inner coating of tin foil, iron filings, 
shot, salt water, or any other conductor, may be used ; the tin foil coating 
is, however, much the most convenient. The jar is charged by connecting 
the knob with the prime conductor, and the outer coating with the earth, 
which latter is the case when the jar is set on an uninsulated table. Here 
the induction is the same as in the Franklin pane; the negative electricity 
passes from the inside of the jar to the prime conductor, the positive fluid 
which remains decomposes the electricity of the outside, driving off the 
positive into the earth. The two sides are then charged with the opposite 
fluids, the inside positive, the outside negative. These conditions will be 
reversed by connecting the outside of the jar with the prime conductor, and 
the inside with the earth. The Leyden jar may also be charged by the 
electrophorus ; whenever all or the greatest part of the combined electricity 
of a jar is decomposed in this manner, It is said to be charged. It may then 
be discharged by means of the discharging rod, by resting one knob of the 
rod on that of the jar, and bringing the other in communication with the 
outside. 
When powerful electrical results are required, it becomes necessary to 
use either very large jars or else a number of jars combined in an electrical 
battery (pl. 20, fig. 37). Here all the outside coatings must be 
connected by resting on a conductor, and all the inside by means of: 
rods. 
The universal discharger of Henley (pl. 20, fig. 38) is a very useful 
instrument for directing the charge of batteries and jars in particular 
directions. It consists of two metallic rods, ending in points, coverable by 
balls, d, f; the rods are insulated by glass pillars. The other extremities of 
the rods carry rings or hooks. ‘There is an insulated table, df, between the 
two posts. ‘To cause the charge of a jar to pass through a given object, it 
is to be placed on an insulated table, and the discharging rods adjusted to 
the proper length. A chain is brought from one rod to the outside of the 
jar, and by means of a glass handle attached to the bail, which tips the 
chain hung to the other rod, a ccemmunication is made with the inside of 
the jar. The interchange of positive and negative electricity from the two 
coatings must pass along the wires, and consequently through the 
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