286 



THE ELECTRIC TELEGRAPH. 



In other words, the sum of those currents which approach 

 the point is equal to the sum of those which recede from it. 

 The truth of this is evident at the first glance ; for, other- 

 wise, the point must be a reservoir, which is contrary to all 

 our notions of electricity. 



The second proposition is that 



" The sum of all the products of the intensities and resistances 

 in all the wires which form an enclosed figure is equal to the sum 

 of all the electro-motive forces in the same circuit." 



A circuit by which this law is illustrated is shown in plan 

 in Fig. 134. E is a galvanic battery, whose circuit divides 



itself, in the points a and 

 b, into the parallel ways 

 r and r 2 respectively. 

 Let the intensities in the 

 three sections of the 

 conductor be I, i lf and 

 i 2 ; according to the law 

 just expressed, the sum 

 Fig. 134. of the product of the 



intensity of the current in each of the branch circuits 

 between a and b, multiplied by its resistance, will equal 

 nothing, since no electro-motive force is found in this cir- 

 cuit, or, 



in *'**= . . . (1. 

 whence, 



that is, the currents in these circuits are inversely proportional 

 to the resistances. 



Further, by the same law, 



I R + 

 I K + 



= E 



and by the first proposition, 



(2. 

 (3. 



By knowing the electro-motive force, E, and the three 



