ELECTRIC LIGHTING. 297 



electric current pass (the current ceasing to pass when the 

 wires are disconnected), in a compound battery, in which 

 (let us say) the metals are zinc and copper, the zinc of one 

 battery is connected with the copper of the next, the zinc of 

 this with the copper of another, and so on, the wire to the 

 copper of the first battery and the wire from the zinc of the 

 last battery being free, and forming what are called the 

 poles of the compound battery -the former the positive 

 pole, the later the negative pole. 1 When these free wires 

 are connected, the current of electricity passes, when they 

 are disconnected the current ceases to pass, unless the 

 break between them is such only that the electricity can, as 

 it were, force its way across the gap. When the wires are 

 connected, so that the current flows, it is as though there 

 were a channel for some fluid which flowed readily and 

 easily along the channel. When the circuit is absolutely 

 broken, it is as though such a cha nel were dammed com- 

 pletely across. If, .however, wliile the poles are not con- 

 nected by copper wires or by other freely conducting 

 substances, yet the gap is such as the electricity can pass 

 over, the case may be compared to the partial interruption 

 of a channel at some spot where, though the fluid which 

 passes freely along the channel is not able to move so freely, 

 it can yet force its way along, with much disturbance and 

 resistance. Just as at such a part of the course of a liquid 

 stream say, a river we find, instead of the quiet flow 

 observed elsewhere, a great noise and lumult, so ; where the 

 current of electricity is not able to pass readily we perceive 

 evidence of resistance in the generation of much heat and 

 light (if the resistance is great enough). 



It will be observed that I have spoken in the preceding 

 paragraph of the passage of a current along the wire con- 

 necting the two poles of a powerful electric b.attery, or along 



1 The extremity of the wire connected with the metal least affected 

 by the acid solution is called the positive pole, that of the wire 

 connected with the metal most affected by the solution is called the 

 negative pole. 



