ELECTROLYSIS, POLARISATION, BATTERIES. 



545 



328. ELECTROLYSIS, POLARISATION, BATTERIES. Electrolysis. Every galvanic 

 current which traverses a fluid conductor causes decomposition or electrolysis of the fluid. The 

 decomposition-products, called "ions," accumulate at the poles (electrodes) in the fluid, the 

 positive pole ( + ) being called the anode [avd, up, dSbs, a way], the negative pole ( - ) the cathode 

 (Kara, down, 68bs, a way). The anions accumulate at the anode and the kations at the 

 cathode. 



Transition Resistance. When the decomposition-products accumulate upon the electrodes, 

 by their presence they either increase or diminish the resistance to the electrical current. This 

 is called transition resistance. If the resistance within the batteiy is thereby increased, the 

 transition resistance is said to be positive ; if diminished, negative. 



Galvanic Polarisation. The ions accumulated on the electrodes may also vary the strength 

 of the current, by developing between the anions and kations a new galvanic current, just as 

 occurs between two different bodies, connected by a fluid medium. This phenomenon is called 

 galvanic polarisation. Thus, when water is decomposed, the electrodes being of platinum, the 

 oxygen (negative) accumulates at the + pole, and the hydrogen (positive) at the - pole. Usually 

 the polarisation current has a direction opposite to the original current ; hence, we speak of 

 negative polarisation. When the two currents have the same direction, positive polarisation ob- 

 tains. Of course, transition resistance and polarisation may occur together during electrolysis. 



Test. Polarisation, when present, may be so slight as not to be visible to the eye, but it 

 may be detected thus : After a time exclude the pri- 

 mary source of the current, especially the element con- 

 nected with the electrodes, and place the free projecting 

 end of the electrodes in connection with a galvano- 

 meter, which will at once indicate, by the deflection 

 of its needle, the presence of even the slightest polarisa- 

 tion. 



Secondary Decompositions. The ions excreted dur- 

 ing electrolysis cause, especially at their moment of 

 formation, secondary decompositions. With platinum 

 electrodes in a solution of common salt, chlorine ac- 

 cumulates at the anode and sodium at the cathode, but 

 the latter at once decomposes the water, and uses the 

 oxygen of the water to oxidise itself, while the hydro- 

 gen is deposited secondarily upon the cathode. The 

 amount of polarisation increases, although only to a 

 slight extent, with the strength of the current, while 

 it is nearly proportional to the increase of the tempera- 

 ture. The attempts to get rid of polarisation, which 

 obviously must very soon alter the strength of the 

 galvanic current, have led to the discovery of two im- 

 portant arrangements, viz., the construction of constant 

 galvanic batteries, and non-polarisable electrodes {du 

 Bois-Reymond). 



Constant Batteries, Elements, or Cells. A perfectly 

 constant element produces a constant current, i. e. , one 

 remaining of equal strength, by the ions produced by 

 the electrodes being got rid of the moment they are 

 formed, so that they cannot give rise to pblarisation. 



lllllllllll l ^l|]M'H|:!NiI';iHi' l |l|IM|i:Mi;i' ! :ri!iii l .;|i^; l iir 



Fig. 381. 

 Large Grove's cell. 

 For this purpose, each of the - substances from the tension series used is placed in a special 

 fluid ( 326), both fluids being separated by a porous septum (porcelain cylinder). 



Grove's cell has two metals and two fluids (fig. 381 ). The zinc is in the form of a roll placed in 

 dilute sulphuric acid [1 acid to 7 of water, which is contained in a glass, porcelain, or ebonite vessel]. 

 The platinum is in contact with strong nitric acid, [which is contained in a porous cell placed 

 inside the roll of zinc]. The 0, formed by the electrolysis and deposited on the zinc plate, forms 

 zinc oxide, which is at once dissolved by the sulphuric acid. The hydrogen on the platinum 

 unites at once with the nitric acid, which gives up and forms nitrous acid and water, thus 



[H 2 + HN0 3 =HN0 2 + H o 0.] 

 [Platinum is the + pole, and zinc the - .] 



[Grove's battery is very powerful, but the nitrous fumes are very disagreeable and irritating ; 

 hence these elements should be kept in a special well-ventilated recess in the laboratory, in an 

 evaporating chamber, or under glass. The fumes also attack instruments.] 



Bunsen's cell is similar to Grove's, only a piece of compressed carbon is substituted for the 

 platinum in contact with the nitric acid. 



[The carbon is the + pole, the zinc the - .] 



[Daniell's cell consists of an outer vessel of glass or earthenware, and sometimes of metallic 



2 M 



