692 INDUSTRIAL APPLICATIONS. 



limit if we make 7=2. On this supposition, and with the above 

 numbers, we get 



p 6-2 



= = 0-174. 



t + 9 25-2 



It is remarkable that this ratio is independent of the nature 

 and price of the electromotor, the luminosity of the lamps forming 

 the compensation. 



1247. GALVANOPLASTICS. The useful work in the galvanic 

 deposition of metals consists solely of an electromotive force to be 

 overcome. In practice this electromotive force is extremely feeble 

 w.hen the positive electrode is formed of the metal to be deposited ; 

 there is then as much dissolved on the one side as is deposited on 



the other, and the solutions retain their strength. The useful effect 

 -p 



u = - increases as the electromotive force of the battery diminishes, 

 ill 



In order to obtain an important effect, without diminishing the 

 efficiency too much, the internal resistance of the battery and the 

 resistance of the connections must be diminished as much as 

 possible. 



1248. ELECTRICAL ARC. Between the carbons which produce 

 the electrical arc there is a fall of potential varying from 40 to 70 

 volts, according to the strength of the current. M. Edlund* has 

 shown, and all experiments have since confirmed, that the greatest 

 part of this fall is due to an inverse electromotive force which 

 may be estimated at 30 volts; the rest is due to the resistance 

 of the interposed layer of gas, a resistance which varies with the 

 length of the arc and its temperature, and is usually from 0*5 to 

 i -5 ohms. 



The brightness of the lamps varies greatly with the density of 

 the current and with the length of the arc, which may be almost 

 null, or may amount to several millimetres that is to say, it 

 depends ultimately on the extent of the incandescent surfaces and 

 their temperature, f 



With a continuous current, the maintenance of a bright arc 

 light, of 100 carcels, requires a current of about 15 amperes, and 



* EDLUND. Ann. de Ckim. et de Phys. [4], Vol. xiu., p. 450; Vol. xiv., 

 p. 493; Vol. xv., p. 479. 1868. 



t ALLARD, LE BLANC, JOUBERT, POTIER, and TRESCA. Comptcs rendus, 

 Vol. xcv., pp. 747, 806. 1882. 



