618 ELECTRICAL CONDENSERS. 



insulated conductors separated by a non-conductor 

 which projects beyond the conductors, are called con- 

 densers or accumulators. 



The two discs are cut out of sheet brass or zinc, 7 or 8 cm in 

 diameter ; any rough edges produced in the cutting must be filed 

 very smooth, and each disc must be hammered quite flat with the 

 mallet ; if brass is used, the discs must first be softened over a lamp 

 or a small charcoal fire. A wire, previously bent as shown in the 

 figure, is soldered to each disc. After washing off the soldering 

 liquid and drying them, the discs are heated until a stick of sealing. 

 wax melts when pressed upon them. The stick of wax which 

 carries the lower disc is fixed at the lower end to a small board 

 which serves as foot. The pendulums are not tied to the wires which 

 are attached to the discs but to small wire hooks, which can be 

 more easily hung into the eyes bent at the ends of the wires and 

 can be conveniently removed again. This mode of suspending the 

 pendulums is the more advisable as in several experiments they are 

 not required ; and as they cause by the fibres of the threads a 

 dissipation of electricity which diminishes the quantity in the 

 apparatus, the pendulums should only be suspended when the ex- 

 istence of free electricity is to be rendered manifest. 



As insulator, a glass plate is best ; if there should be difficulty in 

 obtaining glass which insulates well, a plate of ebonite or sealing-wax 

 must be substituted. Ebonite, especially when dried before the ex- 

 periments, is a very good insulator and is not easily broken, but is 

 more expensive than sealing-wax. The thickness of the plate, of 

 whatever material it may consist, should not exceed 2 mm ; the 

 diameter must be at least 2 cm greater than that of the discs, that is 

 about 9 or 10 cm ; it may be greater but not smaller, for in that case 

 union of the two electricities takes place across the edge of the in- 

 sulator. 



To cast a plate of sealing-wax, 50 or 60 grammes are melted at a 

 very moderate heat in a very small earthen pot or in an old china cup, 

 and the melted mass is poured upon an even surface, when another 

 flat surface is pressed upon it. Two glass plates, or wooden boards 

 which have each been made quite flat with the plane on one side 

 and then covered with tinfoil, will serve for the purpose. The flat 

 surfaces of the plates or boards must be covered with a thin layer 

 of fat, to prevent the sealing-wax from adhering. They are rubbed 

 over with an end of a tallow candle or wiped with a piece of cloth 

 dipped in oil. When the sealing-wax is quite cool, it is removed by 

 pressing sideways against the plates and the wax, for their adhesion 



