Foundations of the Universe 271 



electrons. The reason is that the atoms of zinc which are stolen 

 by the chemical leave their detachable electrons behind them, and 

 the zinc has therefore more electrons to pass on to the copper. 



Such cells are now made of zinc and carbon, immersed in 

 sal-ammoniac, but the principle is the same. The flow of elec- 

 tricity is a flow of electrons ; though we ought to repeat that they 

 do not flow in a body, as molecules of water do. You may have 

 seen boys place a row of bricks, each standing on one end, in 

 such order that the first, if it is pushed, will knock over the second, 

 the second the third, and so on to the last. There is a flow of 

 movement all along the line, but each brick moves only a short 

 distance. So an electron merely passes to the next atom, which 

 sends on an electron to a third atom, and so on. In this case, 

 however, the movement from atom to atom is so rapid that the 

 ripple of movement, if we may call it so, may pass along at an 

 enormous speed. We have seen how swiftly electrons travel. 



But how is this turned into power enough even to ring a 

 bell? The actual mechanical apparatus by which the energy of 

 the electron current is turned into sound, or heat, or light will 

 be described in a technical section later in this work. We are 

 concerned here only with the principle, which is clear. While 

 zinc is very apt to part with electrons, copper is just as obliging 

 in facilitating their passage onward. Electrons will travel in this 

 way in most metals, but copper is one of the best "conductors." 

 So we lengthen the copper wire between the zinc and the carbon 

 until it goes as far as the front door and the bell, which are in- 

 cluded in the circuit. When you press the button at the door, 

 two wires are brought together, and the current of electrons 

 rushes round the circuit; and at the bell its energy is diverted into 

 the mechanical apparatus which rings the bell 



Copper is a good conductor six times as good as iron and 

 is therefore so common in electrical industries. Some other sub- 

 stances are just as stubborn as copper is yielding, and we call 

 them "insulators," because they resist the current instead of let- 



