CHAMBERS'S INFORMATION FOR THE PEOPLE. 



In the following list, the more common elements 

 are arranged in order of their oxidability, or in 

 electro-chemical order : Potassium, sodium, man- 

 ganese, zinc, iron, lead, tin, bismuth, copper, silver, 

 mercury, platinum, gold, hydrogen, carbon, phos- 

 phorus, chlorine, nitrogen, sulphur, oxygen. 



From this we know at once that a zinc-carbon 

 pair will give a current flowing, within the liquid, 

 from zinc to carbon. It follows, from what has 

 been said, that the electro-motive power of any 

 combination will be greater the farther apart they 

 are in this list. For example, a zinc-platinum 

 pair will be more powerful than a zinc-copper 

 one. 



But this is the electro-chemical order only for 

 dilute acids as the exciting liquid. With other 

 liquids, we may reverse the order of the polarity. 

 In a solution of common salt, iron is positive to 

 copper negative ; while in ammonia, or hartshorn, 

 iron is negative to copper positive. For dilute 

 acids generally, however, the order is exactly the 

 same, and such as we have given it above. 



Galvanic Choice of Corrosion. It is a curious 

 fact, that if two metals be placed in contact, in 

 a liquid which can act on both, only the one which 

 is more oxidisable will be attacked. In the gal- 

 vanic pair, the zinc only is eaten away, though the 

 acid would act on the copper if no zinc were 

 present. Hydrochloric acid (or spirit of salt) acts 

 readily on iron, but much more readily on zinc. 

 If, then, a piece of zinc and iron be put in the acid, 

 and made to touch, the zinc will be all dissolved 

 before the iron begin to be corroded. 



Another illustration of the same fact is seen in 

 the action of sulphuric acid on zinc itself. If the 

 zinc of a galvanic pair be pure, the acid scarcely 

 acts on it at all until the circuit is complete ; but 

 if it be impure, then, as soon as it enters the acid, 

 there is a strong effervescence. The reason is, that 

 the other metal particles, such as lead, carbon, 

 iron, &c. present in the zinc form with it just so 

 many closed galvanic circuits, and the zinc wastes 

 rapidly away. This local action, as it is technically 

 called, causes not only a useless waste of zinc, but 

 also a serious enfeebling of the current. Pure zinc 

 would be far too expensive to use for galvanic 

 purposes ; but happily it is found that amalgamat- 

 ing the plate, or rubbing some mercury over its 

 surface, renders the ordinary commercial article 

 as good as the pure zinc for the purpose. 



Comparison of Voltaic and Frictional Elec- 

 tricity. Although the electricity of galvanism is 

 undoubtedly the same natural agency as that of 

 the machine, there is a marked difference between 

 the two, of which it is important to have a clear 

 conception. 



The difference is expressed by saying that, in 

 the voltaic current, we have a large amount or 

 quantity of very feeble electricity ; while, in the 

 electricity of friction, we have great intensity or 

 tension, but no great quantity. 



A very good illustration of the distinction 

 between intensity or tension and quantity is fur- 

 nished by the allied excitement of heat. The 

 temperature of a body may be called its heat- 

 tension, and has very little reference to the 

 absolute quantity of heat-motion or excitement 

 possessed by the body. A red-hot poker, for 

 example, has. heat of a very high tension, though 

 the quantity is very much less than that of a 

 warm-water bath. 



270 



Nor need we be surprised that the properties of 

 voltaic and frictional electricity should differ so 

 widely. The difference is not perhaps greater 

 than that of the powers and properties which 

 belong to heat of weak and of strong intensity. 

 A small wire, made red hot, will suffice to burn 

 the finger or explode gunpowder, though an 

 infinite quantity of heat applied at low tension has 

 no such effect. 



In modern language, the electricity of the 

 current is kinetic electric energy ; that of the 

 machine is potential electric energy. The former 

 or actual motive form of the force is electric 

 energy in the course of transformation into another 

 form. It will appear wholly as heat in the circuit, 

 if it be used to do no -work; or it may appear 

 partly as heat and partly as chemical or mechanical 

 energy. The sum of the new forms of energy is 

 always an exact equivalent of the primary energy 

 of the current. The potential energy of frictional 

 electricity is, like that of magnetism, power in 

 store, ready to be transformed into kinetic energy, 

 such as attraction, when suitable circumstances 

 arise. 



GALVANIC BATTERIES. 



When a number of galvanic pairs, such as 

 we have described, are put together, so that 

 the currents given by each shall all unite and 

 flow in the same direction, they form a galvanic 

 battery. The copper plate of each cell (fig. 21) 

 is connected, by a strip of copper, with the zinc 

 of the next, so that we have a zinc plate left 

 alone at one end, and a copper at the other. 

 These are called the poles of the battery, and the 

 circuit is completed by joining the poles with a 

 wire or conductor. The group thus acts exactly 

 as a compound galvanic couple, the copper plate 

 being the positive pole, and the zinc the negative. 

 In the outside parts of the circuit, the direction of 

 the current is always from the copper to the zinc, 

 and in the inside or liquid parts, from the zinc 

 through the liquid to the copper. By thus joining 

 several couples, the polarisation of the first copper 

 is transmitted to the second zinc by the connecting 

 wire, and serves to heighten the effect of liquid 

 number two in polarising zinc and copper number 

 two. Obviously, therefore, the electro-polarising 

 force of each cell serves to increase that of the 

 next ; and the result is, that at the final pole C, 

 the electro-motive force or power to set up polarity 

 along the wire leading from it is very great. It is 

 just as many times greater than that of a single 

 cell as there are cells so linked together. 



We must, however, carefully distinguish between 

 the electro-motive force of a battery and the strength 

 of current which it transmits. In certain circum- 

 stances, one of the cells will give as strong a 

 current as a hundred joined together as a battery, 

 though the electro-motive force of the former is a 

 hundred times smaller. Thus, the strength of the 

 current is not to be confounded with the strength 

 of the cell or battery. The strength or electro- 

 motive force of a battery (or cell) is estimated by 

 the amount of statical charge in the ends of the 

 wires, before the circuit is joined that is to say, 

 by the amount of attraction or repulsion they can 

 produce. So measured, then, the strength of a 

 battery may be defined as its power to propagate 

 electricity, or push forward the polarisation against 



