466 Mr. Snow Harris on Liglitning Conductm's 



coating, previously to making the discharge ; the spark will 

 then appear to ensue at the time of the discharge. 



(o). Examine the jar after this residuary spark has been 

 taken from the outer coating, and it will be found again slightly 

 charged as at first. 



(jp). Charge a jar, exposing about two square feet of coat- 

 ing, with a given quantity of electricity, measured by the unit 

 jar u, fig. 5. Let a conducting rod terminating in a ball ?•, 

 project from the outer coating, and place near it the electro- 

 scope E*. Discharge the jar through the rod c c as before, 

 and observe the amount of divergence of the electroscope. 

 Double the capacity of the jar, and again accumulate and 

 discharge the same quantity. The divergence of the electro- 

 scope will be very considerably decreased. Add a second 

 and a third jar to the former, and the effect will be at last 

 scarcely perceptible : connect the jar with the ground, and 

 with a given quantity the spark will vanish altogether. 



[q). Accumulate a given quantity as before, and observe 

 the effect of the residuary charge on the electroscope. Let 

 a double, treble, &c., quantity be accumulated and discharged 

 from a double, treble, &c., extent of surface ; that is to say, 

 for a double quantity employ two similar jars, and so on : the 

 effect will remain the same. 



(r). The quantity and surface remaining constant, let the 

 discharge be effected by discharging circuits c c fig. 3, of 

 different dimensions from a large rod down to a fine wire 

 which the charge in passing can make red-hot. Observe the 

 effect on the electroscope in each case : it will be found nearly 

 the same, being rather less where the tension in the dischar- 

 ging wire is very considerable. 



(s). Connect the jar with the ground, and place between 

 the discharging conductor c fig. 3. and a metallic mass B, a 

 small quantity of percussion powder, inclosed in thin paper. 

 The powder will not be inflamed, even in the case of the dis- 

 charging conductor becoming red-hot; whereas in passing 

 the slightest spark, it inflames directly. 



{t.) Insulate a circular conducting disc, M, fig. 6, of four 

 feet in diameter : it may be made of wood covered with tin 

 foil ; oppose to it a similar disc, N, connected with the ground. 

 Place a conducting rod, c c, on the lower plate, and near it a 

 metallic body, o ; electrify the upper plate, m ; dense sparks 

 will fall on the rod, c c, but no effect is observable on the vi- 

 cinal body, o, even though percussion powder be placed in the 

 opening. 



* The electroscope I employed is described in the Transactions of the 

 Royal Society for 1834, Part 2, page 214. For more accurate measurement 

 we should employ the electrometer, p. 215. 



