40 



ELECTRICITY. 



tended the whole length of Westminster 

 bridge, at a considerable height above 

 the river: one end of this wire com- 

 municated with the outer coating of a 

 charged phial, the other being held by a 

 person on the opposite side of the river, 

 who formed a communication with the 

 water by dipping into it an iron rod 

 held by the other hand. The circuit 

 was completed by another person, who 

 stood near the phial, and who likewise 

 dipped an iron rod into the river with 

 one hand, and was enabled, by means 

 of a wire held in the other, to effect a 

 contact with the knob of the phial. 

 Whenever the discharges took place, the 

 shocks were felt by both persons ; thus 

 proving that the electric fluid must have 

 been in motion along the whole line of 

 the circuit, including both the wire 

 above and the river below, 



In another experiment, made on Shoot- 

 ers'-hill, at a time when the ground was 

 remarkably dry, the electricity was made 

 to perform a circuit of four miles ; being 

 conducted for two miles along wires 

 supported upon baked sticks, and for 

 the remaining distance, also of two miles, 

 through the dry ground. As far as 

 could be ascertained, by the most careful 

 observation, the time in which the dis- 

 charge was transmitted along that im- 

 mense circuit was perfectly instantane- 

 ous : nor has any other trial that has 

 yet been made afforded the least ap- 

 proach to a measurement of the velocity 

 with which electricity moves. 



(142.) On this subject, however, an 

 important distinction should be made 

 between the actual movement of each 

 individual particle of electric fluid, and 

 the transmission of an impulse along a 

 series of such particles, for the one may 

 bear hardly any proportion to the other : 

 just as we find that sound proceeds with 

 a velocity incomparably greater than 

 that of the particles of air which are 

 concerned in its propagation. In like 

 manner the portion of blood, which raises 

 the artery at the wrist, where the pulse 

 is felt, is not the identical portion of 

 blood which is thrown out from the heart 

 by the contraction of that organ pro- 

 ducing that pulsation : the impulse, in 

 all these cases, being propagated like a 

 wave, from one particle to another. 

 There is, therefore, no reason to sup- 

 pose that the same particles of electric 

 fluid, which enter at one part, have tra- 

 versed from one end to the other the 

 whole line of conducting substances 

 which form the circuit. 



(143.) If we conceive the conducting 

 bodies which compose the circuit to be 

 divided into an indefinite number of 

 filaments, every one of which is capable, 

 in an equal degree, of conveying the 

 electric fluid, it is evident that the united 

 power of these filaments, or what is 

 the same thing, the capability of the 

 body itself to convey a charge of elec- 

 tricity, is in proportion to the number 

 of these elementary filaments which it 

 contains, that is, to the magnitude of its 

 transverse section, without any relation 

 to its form. Thus, the same metallic 

 rod will conduct a charge equally well, 

 whether it be flattened, or divided into 

 several smaller wires, or whether it con- 

 sist of a single cylinder of the same 

 area. 



(144.) If the size of the conductor be 

 sufficiently great, the whole charge may 

 be conveyed without any sensible ob- 

 struction or retardation, and therefore 

 without any tendency to deviate from 

 the direct line of its course. But it is 

 otherwise when the conductor is too 

 slender to afford a ready passage to the 

 fluid which is pressing onwards : and it 

 is important to inquire into the conse- 

 quences to which these obstructions may 

 give rise. 



(145.) The first effect of an impedi- 

 ment to the free passage of accumulated 

 electricity must be a retardation of its 

 motion-. It is reasonable, therefore, to 

 expect that with a circuit composed 

 either of bad conductors, or of con- 

 ductors of inadequate size, although 

 good, the discharge will not be effected 

 so instantaneously, nor so completely ; 

 and that the shock which accompanies it 

 will be diminished in its violence. This 

 principle may find its application on oc- 

 casions where it is desirable to soften 

 the intensity of 'the shock, as in the 

 medical employment of electricity, where 

 imperfect conductors are on this ac- 

 count sometimes preferable, both for 

 taking sparks and shocks. 



(146.) A second eftect resulting from 

 an obstruction to the flow of electricity, 

 is a tendency in the fluid to diverge 

 from the direct line of its course, and to 

 fly off to different objecis in the vicinity. 

 This is frequently exemplified in the 

 case of lightning, which, on striking a 

 building, is apt to take a very irregular 

 and seemingly capricious route, darting 

 towards conducting bodies which may 

 happen to attract it, although at some 

 distance from the immediate direction it 

 was pursuing. The position of such 



