294 ELEMENTS OF ELECTRICAL ENGINEERING. 



any lamp in the group, and the size of the wires, efab and gcd, is 

 usually determined with reference to the voltage-drop between the 

 service point, eg, and the middle lamp, p. Let / be the total cur- 

 rent delivered to the group of lamps, and let the lamps be assumed 

 to be uniformly distributed as shown in Fig. 164^; then the cur- 

 rent in the element, A,r, is I(X x)/X, and the voltage-drop in the 

 element, A;tr, is p A;r times I(X *)/X, where p is the resistance 

 per unit length of the wire, cd. Therefore the voltage-drop along 

 cd from c to the middle lamp is : 



' 



Also the voltage-drop along ab from a to the middle lamp is 

 y% pXI, so that the total drop between the service point, eg t and 

 the middle lamp, /, is I(r' -f r"), where r' is the resistance of gc 

 plus the resistance of efa, and r" is equal to "fy^pX, where pX is 

 the resistance of one of the wires, ab or cd. 



Rule 3. To give a prescribed voltage-drop between the ser- 

 vice point and the middle lamp of a row, which is connected 

 according to the return loop scheme, make the wire of such size 

 that the total current delivered to the group of lamps would give 

 the prescribed drop over a length /' -f 1" of the wire, where /' is 

 the sum of the distances,^ and efa, in Fig. 164^, and I" is ^ of 

 the distance ab. 



