Electrical Discharge. 347 



T = p, or T= — . I have shown, for example, in my communica- 

 tions to the Philoso^ihical Transactions*, that with circuits of cop- 

 per wire varj'ing from 300 to 900 feet in length, arranged in a zig- 

 zag form upon insulating supports, the effect of a given quantity 

 of electricity discharged through the wire of the thermo-electro- 

 meter is inversely proportional to the length of the circuit, the cir- 

 cuit in this case being taken in lengths of 300, 600, and 900 feet, 

 a result which M.Riess has himself confirmed by a subsequent and 

 similar experiment ; hence my expression for the heating effect of 



Q2 



the discharge (1) becomes T= — . Now it is important here to 



observe, that this expression T= — is virtually the same as M. 



Q2 r ■' 



Riess's expression T= — , the symbol s being, when correctly 



interpreted, nothing more than the resistance we necessarily in- 

 troduce into the circuit of discharge, in augmenting the number 

 of charging rods and other obstacles, when we extend our battery 

 by increasing the number of jars ; to which we must add the 

 resistance arising fi-om a division of the coated glass upon which 

 the charge is accumulated. In order to make an exact expei'i- 

 ment, we should accumulate and discharge the same quantity of 

 electricity, either from a given number of jars of different amount 

 of coated surface, or otherwise from a single jar in which the 

 extent of the coating may be varied, or otherwise from coated glass 

 of variable thickness. In either case we may change the indicated 

 " density," " determinee directemcnt au moyen d'un electrometre 

 k poids," without changing the resistance in the circuit. In all 

 these cases, however, although the electrometer greatly varies, 

 the heating effect of the discharge remains the same. If M. De 

 la Rive and j\I. Ricss had resorted to experiments of this kind, 

 they would have found the heating effect quite independent of 

 what they term " density" of the electricity in the battery as mea- 

 sured by a balance electi'ometer, or otherwise estimated by the 

 extent of the battery. 



1 1 . Take, for example, two jars. A, B, fig. 8, one having about 

 a square foot and a half of coating, the other five or six square 

 feet, charge them with the same quantity of electricity, and 

 then discharge each in succession through the wire of the thermo- 

 electromcter E, taking care to employ in each case the same 

 charging rods and circuit. The heating effect will be the same, 

 or very nearly so, notwithstanding that the " density," as indi- 

 cated by the electrometer, may with the small jar A be sixteen 



* Trans. Roy. Soc. for 18.'i4, p. 228, 

 2 A 2 



