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OUR PHYSICAL WORLD 



than glass, which scarcely permits any electricity to flow through 

 it, and so is called a non-conductor. Recall the heat conductivity 

 of these substances, page 153. Furthermore, if a pipe carrying 

 water branches, the flow in each branch will be in proportion to 

 its capacity; if one branch has a cross-sectional area twice that 

 of the other, it will carry twice as much water. Similarly, if a 

 wire carrying a current branches, the flow of current in each 

 branch will be proportional to its capacity; if the circuit supplied 

 by one branch offers much resistance, while the other offers little, 

 the latter will carry the major part of the current. 



FIG. 94. Diagrams of batteries connected (a) in series and (b) parallel, and 

 of water tanks to correspond. 



Batteries are said to be connected in series when the positive 

 plate of one is connected by a wire with the negative plate of the 

 next. One of the terminal wires of the series will come from a 

 positive plate, the other from a negative. Batteries are said to 

 be connected parallel when the positive plates of all are connected 

 by one wire, and the negative plates of all are connected by 

 another wire. When two or more batteries are connected in 

 series, the effect is similar to that of mounting one water-tight 

 reservoir above another and connecting them by pipes. The 

 pressure of water in the upper tank is added to that of the lower, 



