36 ELECTRIC LIGHTING. 



It follows from this mode of action that when a complete 

 circuit is closed near the battery, the current at first sets -out 

 from the two poles of the battery, and produces its effect in 

 the centre of the circuit last; so that at the first instant one- 

 half of this circuit is traversed by a positive charge, and the 

 other half by a negative charge. But this takes place only 

 when the circuit is wholly metallic. If the earth is inter- 

 posed the positive charge traverses the whole length of the 

 wire, for the negative charge is then completely absorbed by 

 the earth. 



For a long time it was supposed that the charge of a circuit 

 was in a manner instantaneous, and that it had its limit of 

 velocity only in that of electricity itself, a velocity which was 

 believed to approximate to that of light ; but a deeper study 

 of the mode of propagation of electricity showed that this 

 was far from being the case, and that, in fact, properly speak- 

 ing, there was no such thing as a velocity of electricity, but 

 rather a period of electric fluctuations during which each 

 point of the circuit incessantly changes its electric tension. 

 Hence we were led to assimilate the propagation of elec- 

 tricity to that of heat, and for the first time were able to form 

 a clear conception of the effects of electric transmission on 

 our long telegraphic lines. 



It was in 1825 that the illustrious Ohm made the dis- 

 covery upon which he founded the theory which bears his 

 name, and which modern discoveries have only served more 

 fully to justify. Nevertheless, this theory was not at first 

 received by the scientific world with that favour which its 

 author had a right to expect ; he was on account of it even 

 subjected to a persecution that compromised his position as 

 professor ; and it was only ten years later, when Pouillet had 

 arrived at the same laws by experiment, that people began 

 to revise the sentence they had pronounced against Ohm 

 and to appreciate the merit of his discovery. Yet while 

 adopting the formulas of the illustrious mathematician, 

 physicists were, until the year 1860, unwilling to accept the 



