366 On the Intensity and Qtmntity of Electric Currents. 



infinite. In Zamboni^s pile it is extremely great. In hydro- 

 electric and magneto-electric circuits, intended to give so-called 

 intense currents, the number of the elements or of the windings 

 of the coil must be augmented, the necessary resistance of coui*se 

 increasing in the same proportion. 



On the contraiy, all those cun'ents to which a moderate or a 

 small intensity, in proportion to the quantity, has been attributed, 

 arise from sources of small necessaiy resistance. This is the 

 case, for example, with thermo-electric currents. And in order 

 to increase the quantity of hydro-electric, or magneto- electric 

 currents in proportion to their intensity, the size of the cells in 

 the first case, or the diameter of the wire in the other case, must 

 be increased. 



If two circuits are taken of very unequal necessary resistances 

 with electromotive forces diiFering in the same proportion as the 

 necessary resistances, whilst the accidental (external) resistances 

 are imperceptible or also proportional to the necessary resistances, 

 then the intensity (strength) of the currents in these two circuits 

 will be exactly the same, and will remain so as long as the elec- 

 tromotive force, the necessary and the accidental resistances, 

 vary in the same proportion. This is exemplified in the case 

 first mentioned, in which a hydro-electric and a thermo-electric 

 current produce the same efiect on the magnetic needle. This 

 occurs notwithstanding the greater necessary resistance of the 

 hydro-electric circuit, because of its greater electromotive force. 

 That the electromotive force is greater is easily demonstrated by 

 making the cun'ents from the two sources oppose one another in 

 the same circuit, when the current of the hydro-electric source 

 will be paramount. It is evident that in this case the sun^ of 

 the resistances which each current has to overcome is the same. 



Thus, according to Ohm's theory, two currents, although 

 produced by sources of different electromotive force, may be 

 quite identical in their action, provided the resistances of the 

 two circuits bear to each other the same proportion as the t\^^o 

 electromotive forces. It will, however, be found that an immense 

 difference exists in two such circuits whenever the proportion of 

 the resistances is changed, or whenever an additional resistance 

 is added to both circuits. When the necessary resistance of one 

 of the circuits is very large, a considerable additional resistance 

 may be added to it without any great change in the whole sum 

 of the resistances being produced ; accordingly the current will 

 not experience any considerable loss of intensity, the magnetic 

 needle will be deflected almost as much as it was before, and a 

 conducting body introduced into the circuit will be acted upon 

 with energy. In short, the current firising from a source of 

 great necessary resistance will seem to overcome easily the 



