38 ELECTRIC LIGHTING. 



nothing but the result of the tendency to equilibrium, from 

 one end of the circuit to the other, of two. different electrical 

 states, determined by the action of the battery, and repre- 

 senting, therefore, the two different temperatures of the heated 

 bar. This variable period must no doubt be excessively 

 short on account of the subtilty of the electric fluid, but 

 with circuits of great length and with those formed of bad 

 conductors it may be measured, and this, in fact, Gaugain 

 found by experiment. He thereupon investigated the laws 

 of the transmission of the current during this variable period, 

 and, among other laws, he found that the time required for 

 a current to attain its permanent condition in a circuit, that 

 is, to acquire all the intensity of which it is capable, is pro- 

 portional to the square of the length of the circuit. This had 

 not only been foreseen by Ohm, but had even been mathe- 

 matically formulated by him in the equation representing 

 the tensions of the different points of a circuit during the 

 variable period of the current's intensity. 



THERMO-ELECTRIC GENERATORS. 



The thermo-electric piles invented by Seebeck in 1821 

 had for a long time been regarded merely as generators of 

 great constancy, capable of being very advantageously used 

 in scientific experiments, but incapable, on account of the 

 weakness of their currents, of being applied in practice. 

 But the application which, some years ago, Marcus made of 

 the considerable thermo-electric power of metallic alloys, * 



* The discovery of the great thermo-electric power of metallic alloys was 

 made in the first instance by Seebeck, who even mentions the alloy of anti- 

 mony and zinc as one of those which might be most advantageously used ; 

 but these thermo-electric systems were not applied until ten years afterwards, 



