366 HOW .ELECTRICITY IS CONTROLLED 



When we join our three tanks of water at the same level, 

 we increase the capacity to furnish a current of water, 

 although it has no greater pressure than the single tank. 

 We may join three cells in parallel by connecting all the 

 zinc poles to one wire, and all the carbon poles to another 

 wire, then we. join these two wires where we wish to use 

 the -current. If we attach a single cell to the voltmeter 

 and read the voltage, and then later attach three cells 

 joined in parallel to the voltmeter and read the voltage, 

 we find it to be the same. Any number of cells joined in 

 parallel have just the same voltage as a single cell. With 

 the same size of opening for the single and the triple water 

 tanks the water would flow from the triple tank for a longer 

 time. It has a larger reservoir, so with the cells joined in 

 parallel, if the voltage of one cell gives all the pressure you 

 need, you can get much longer service from several cells 

 joined in parallel than you can from one cell. These 

 explanations should serve to make it possible for you to 

 now use the batteries in your own home intelligently. 



The life of a cell. If you have a large outlet to a tank 

 of water, it will quickly drain the tank. The less re- 

 sistance you have in the circuit of an electric cell, the 

 greater will be the flow of the current, and since the ca- 

 pacity of a cell is limited, its life will be shortened. The 

 practice of connecting two poles of a cell by a short piece 

 of metal quickly spoils the cell. If you wish your cell to 

 work for you for a long time, be sure not to allow any 

 " short circuits." 



What is a "short circuit"? We have all used the 

 term " short circuit," and now let us try to understand 

 fully what it means. The circuit of electricity is the 

 complete path which the current must take from the cell 



