Electrical Discharge. 345 



lating the altitude of the spirit in the stem, and used as small a 

 quantity of spirit as possible*. Here is, as must be allowed, very 

 strong evidence as to the sensibility of my instrument at least ; 

 and upon the whole I am led to doubt the great superiority and 

 advantage claimed by M. Riess for the particular construction of 

 my instrument which he employed, and which is figured by M. 

 De la Rive in his recent work, vol. ii. p. 156. 



7. Having thus explained and illustrated the application and 

 use of the thermo-electrometer, and which was invented full 

 thirty-five years since, I shall endeavour, for the objects of 

 science, to show its exact accordance with the formula of M. Riess 

 when correctly interpreted, as well as its great applicability to 

 the purposes of electrical research. In order, however, to avord 

 any misapprehension, we will revert first, in express terms, to the 

 question we are about to consider. My announcement was, that 

 the heat excited in a metallic wire by the electrical discharge is 

 always the same for the same quantity of electricity, whatever 

 may be the intensity indications of the common electrometers 

 placed in connexion with the battery. M. le Professeur Riess says, 

 on the other hand, that this announcement is " untrue," that 

 he has found the heating eflPect of the discharge inversely pro- 

 portional to the extent of the battery upon which the electricity 

 is accumulated, that is to say, proportional to the product of the 

 quantity by its " density." Let us here pause for a inoment to 

 consider what we are really to understand by the term " density " 

 of the electricity accumulated in a battery, and which we ima- 

 gine to be measured by the ordinary " intensity " electrometers f. 



8. If we rigorously examine this very hypothetical question, 

 we shall find that these instruments do not really furnish us with 

 any information whatever relative to "tension" or "density" of 

 the accumulated electricity at the instant of discharge ; that is to 

 say, at the instant in which the accumulation coming from every 

 point of the coated glass, falls, as it were, in a concentrated form 

 upon the metallic wire, the subject of experiment ; neither do 



* The best liquid for ordinary purposes is plain distilled water, coloured 

 by a little tincture of cochineal. Spirit is liable to damage the cemented 

 metallic portions of the instrument. 



t " Outre la quantite d'electricite, il importe de connaitre sa density, 



laquelle depend de I'etendue s de I'armure interieure de la batterie 



Pour une meme quantite d'c'lectricite cette densite est inverse de cette 

 ^tendue, de sorte qu'on peat Texprimer par la fraction ^. La density pent 



etre deter minie directement au moyen d'un ^lectrom'etre a poids, Sfc." — De 

 la Rive, Tralte de I' Electricity, vol. ii. p. 159. The author gives a figure 

 of the balance-electrometer employed by M. Riess, as figured in his work, 

 Reihunys-elekfricitiit, vol. i., and by which he infers the density of the 

 charge. 



P/til. May. S. 4. Vol. 11. No. 73. May 185G. 2 A 



