130 ELEMENTARY LESSONS ON [CHAP. in. 



greatest electromotive-force, or be the most " intense," 

 in which those materials are used which give the 

 greatest difference of potentials on contact, or which are 

 widest apart on the " contact-series " given in Art. 72 

 Zinc and copper are very convenient in this respect 

 and zinc and silver would be better but for the expens< 

 For more powerful batteries a zinc-platinum or a zin 

 carbon combination is preferable. 



158. Resistance. The same electromotive -fon 

 does not, however, always produce a current of the sair 

 strength. The strength of the current depends not only o 

 the force tending to drive the electricity round the circui 

 but also on the resistance which it has to encounte 

 and overcome in its flow. If the cells be partly choke 

 with sand or sawdust (as is sometimes done in s< 

 called " Sawdust Batteries " to prevent spilling), or, if th 

 wire provided to complete the circuit be very long < 

 very thin, the action will be partly stopped, and tl 

 current will be weaker, although the E.M.F. may 

 unchanged. The analogy of the water-pipes will aga; 

 help us. The pressure which forces the water throug 

 pipes depends upon the difference of level between th 

 cistern from which the water flows and the tap to whic 

 it flows ; but the amount of water that runs through wi 

 depend not on the pressure alone, but on the resistanc 

 it meets with ; for, if the pipe be a very thin one, ( 

 choked with sand or sawdust, the water will only ru 

 slowly through. 



Now the metals in general conduct well : their resisj 

 ance is small ; but metal wires must not be too thin < | 

 too long, or they will resist too much, and permit Ally 

 feeble current to pass through them. The liquids in tl 

 battery do not conduct nearly so well as the metal 

 and different liquids have different resistances. P' 1 

 water will hardly conduct at all, and is for the^le 

 electricity of the voltaic battery almost a perffn- 

 sulator, though for the high-potential electricity he 



