516 



LECTURE LIV. 



ON ELECTRICITY IN MOTION. 



THE manner in which the electric fluid is transferred from one body to 

 another, the immediate effects of such a transfer, the causes which origi- 

 nally 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 excitation by which the equilibrium is originally disturbed, 

 one of the most interesting 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 chemical than to the mechanical doctrine of 

 electricity. 



The progressive motion of the electric fluid through conducting sub- 

 stances 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 pass to a very great distance, a perceptible portion of 

 time is actually occupied in 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 substance, 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 density of the fluid from the velocity of a spark or shock, 

 compared with that of sound ; or we might deduce its velocity from a 

 determination of its density. It has been supposed, although perhaps 

 somewhat hastily, that the actual velocity is nearly equal to that of 

 light* 



When a conducting substance approaches another, which is electrified, 

 the distribution of the electric fluid within it is necessarily altered by 

 induction, before it receives a spark, so that its remoter extremity is 

 brought into a state similar to that of the first body : hence it happens 

 that when the spark passes, it produces less effect at the remoter end of 

 the substance, while the part presented to the electrified body is most 



* Watson's Exp. to determine the Celerity of Electricity, Ph. Tr. 1748, pp. 49, 

 491. Wheatstone, ibid. 1834, p. 583. 



