292 ELEMENTS OF ELECTRICAL ENGINEERING. 



of course be kept within bounds by limiting the voltage-drop be- 

 tween the service point and the end lamp of the group. In fact 

 a group of lamps which is to be operated as a unit is generally 

 wired according to the same rules as a group that is not to be 

 operated as a unit, that is the wiring is generally laid out accord- 

 ing to rules i and 2 of Case I.; but a special wiring scheme, 

 called the return loop scheme,* may be used to eliminate voltage 

 variations of the first type (a) in a group of lamps that is operated 

 as a unit, whatever the total voltage-drop may be. 



The fundamental idea of the return loop scheme may be seen 

 with the help of Fig. 1640. The current in the wire, ab, at any 



point, />, is proportional 

 to the distance, pb, the 

 lamps being assumed to 

 - be uniformly distributed ; 



p J and the current at any 



point, /', in the wire, cd, 

 is proportional to the dis- 



- X tance, p'd. If the wires, 



ab and cd, are tapered 

 so as to have sectional 



Fig. 164*. 



areas proportional to the 



current at each point, then the value of Ri is the same in 

 both wires between any pair of lamps ; but Ri is a drop of vol- 

 tage in a given direction along one wire and a rise of voltage in 

 the same direction along the other wire, therefore the lamp vol- 

 tage is constant throughout the group of lamps, whatever the 

 total voltage-drop between the service point and the lamps may be. 

 The use of tapered wires is of course impracticable and the 

 return loop scheme is always carried out either with wires tapered 

 in steps or with wires of uniform size, usually the latter. Under 

 such conditions the voltage varies to some extent from lamp to 

 lamp but the range of this variation is very much less than the 

 total drop. 



* Sometimes called the anti-parallel scheme. 



