668 



LECTURE LIV. 



ON ELECTRICITY IN MOTION. 



XHE manner in which the electric fluid is transferred from one body to an- 

 other, the immediate effects of such a transfer, the causes which originally 

 disturb the equilibrium of electricity, and the practical methods, by which all 

 these circumstances are regulated and measured, require to be considered as 

 belonging to the subject of electricity in motion. Among the modes of ex- 

 citation by which the equilibrium is originally disturbed, one of the most inter- 

 esting is the galvanic apparatus, which has been of late years a very favourite 

 subject of popular curiosity, and of which the theory and operation will be 

 briefly examined, although the subject appears rather to belong to the che- 

 mical than to the mechanical doctrine of electricity. 



The progressive motion of the electric fluid through conducting substances 

 is so rapid, as to be performed in all cases without a sensible interval of time. 

 It has indeed been said, that when very weakly excited, and obliged to pas« 

 to a very great distance, a perceptible portion of time is actually occupied ia 

 its passage; but this fact is somewhat doubtful, and attempts have been made 

 in vain, to estimate the interval, employed in the transmission of a shock 

 through several miles of wire. We are not to imagine that the same particles 

 of the fluid, which enter at one part, pass through the whole conducting sub- 

 stance, any more than that the same portion of blood, which is thrown out of 

 the heart, in each pulsation, arrives at the wrist, at the instant that the pulse is 

 felt there. The velocity of the transmission of a spark or shock far exceeds 

 the actual velocity of each particle, in the same manner as the velocity of a 

 wave exceeds that of the particles of water concerned in its propagation; and 

 this velocity must depend both on the elasticity of the electric fluid, and on the 

 force with which it is confined to the conducting substance. If this force 

 were merely derived from the pressure of the atmosphere, we might infer the 



