TRANSMISSION OF ELECTRICITY BETWEEN CONDUCTORS. 225 



direction of the opposed substance is continually increasing ; and at length, in virtue 

 of its connexion with the mass of the earth, if it be in a free state, indefinitely great. 

 This reasoning applies also to the discharge of an electrical accumulation between the 

 coatings of a jar, the force in the direction of the discharging circuit, being at the 

 instant of the discharge indefinitely great, in respect of the controlling force exerted 

 on the accumulated electricity by the metallic coatings, taken either singly or as 

 acting one on the other through the intervening glass. 



31. The phenomena of tension and intensity as above explained, are quite inde- 

 pendent of the eflfect of the whole quantity accumulated, when discharged through 

 various substances. Thus, the heating effect of a given quantity, discharged through 

 a metallic wire, under the same conditions of circuit, &c., is always the same, whatever 

 may have been its previous tension or intensity, as relating to the conductors on 

 which the accumulation has taken place ; e. g. 3, given quantity, accumulated on 

 coated jars, always produces the same heat in a metallic wire, c d, fig. 13, inclosed in 

 the bulb of the electro-thermometer N*, and discharged by means of the drop-ball/* 

 whether accumulated on thick glass, or on thin, or on a greater or less extent of sur- 

 face, the number of jars and the length of the circuit being the same. Dr. Faraday, 

 in his capital researches in magneto-electricity, has further shown, that the same 

 is true in respect of the magnetic effects produced, as also in respect of the electro- 

 chemical effects ; we have therefore arrived at a distinguishing property of quantity, 

 of great consequence to inquiries in this branch of science. 



32. The circumstances attending the transmission of a momentary electrical current 

 between two conductors, under the form of a dense explosion, merits, in relation to 

 the above deductions, an attentive consideration. 



When the attractive force operating between two conductors can overcome the 

 atmospheric pressure, a discharge ensues between the nearest points of the opposed 

 surfaces. In these points the force appears to become at length indefinitely great, in 

 respect of points more remote, so that the whole quantity accumulated, is finally de- 

 termined through them. Thus, the precise points of contact between two spheres 

 being found, and the spheres subsequently separated by given distances measured 

 between these points, it may be shown, that the respective quantities requisite to pro- 

 duce a discharge will vary with the distances directly. 



{h) A discharging electrometer, fig. 14, was so constructed that given distances 

 might be obtained between the nearest points of the spheres c c' by means of a micro- 

 meter screw, s. This instrument being affixed to a jar, D, exposing about five square 

 feet of coating, it was easy to estimate very exactly by means of the unit jar u (11.) 

 the quantity of electricity requisite to cause a discharge at any given distance between 

 the balls c c\ Under these circumstances it is found that the number of measures in- 

 dicated by the unit jar, vary exactly with the distances between the nearest points of 



* For a description of this instrument I may refer to the Philosophical Transactions for 1827, p. 18 ; also to 

 the Transactions of the Royal Society of Edinburgh for 1832. 



