174 



KNOWLEDGE. 



Septeubeb 1, 1891. 



problem. But it is, however, possible that two stars of 

 similar colour might have different spectra, because there 

 are several pairs of complementary tints which, when 

 united, produce white light. 



W. S. Franks. 



SOME PRACTICAL APPLICATIONS OF 

 ELECTRICITY. 



By J. J. Stewakt. 



(Continued from paije 33.) 



n. — Secondajry Batteries. 



WHEN a current of Electricity passes fi-om a 

 copper plate immersed in a solution of a copper 

 salt, such as copper sulphate, to another plate 

 opposite to it in the same solution, say one 

 made of platinum, copper is deposited on the 

 platinum plate owing to the decomposition of the solution, 

 the amount of copper thus set free being proportional to 

 the strength of the cm-rent which passes. Faraday was 

 the first to prove this, and called the apparatus, consisting 

 of the trough of solution and the plates dipping into it, a 

 roltiiineter, because it served as a means of measuring the 

 strength of currents. [I may say, in passing, that a 

 voltameter on just the same principle as this is used by 

 Edison to act as a current meter to indicate the quantity of 

 Electricity sent from his electric light mains to any single 

 house. ^ If an electric current is made to traverse a ceU 

 consisting of two plates of platinum placed opposite each 

 other in acidulated water, the water is continuously de- 

 composed into its constituent elements during the passage 

 of the current, provided this cm-rent is above a certain 

 strength ; but whilst this operation goes on a remarkable 

 phenomenon is noticed, the current which commences with 

 a given strength becomes weaker and weaker, and if a 

 galvanometer is placed in the circuit in order that the 

 behaviour of the cm-rent may be observed, what is seen to 

 happen is this — the galvanometer needle, when the current 

 through the cell is first started, swings ofl' vigorously and 

 comes to rest at a certain angle of dedection, indicating the 

 strength of the current, but, on fm-ther watching, it is seen 

 to be creeping back ; the deflection is gradually decreasing 

 towards zero. On looking at the platinum plates in the 

 water, bubbles are seen to be formed upon their surfaces, 

 which rise in two streams to the top of the liquid ; 

 these consist of the constituent gases of the water, 

 hydrogen being set free at the plate where the current 

 leaves the voltameter cell — or the cathode, as Faraday 

 calls it — and oxygen at the plate where the current 

 enters, or the anode ; some of these gas bubbles, however, 

 are seen to cling to the plates instead of rising up through 

 the water. On looking more closely, it is seen that the 

 streams of rising bubbles are decreasing simultaneously 

 with the falling off of the galvanometer deflection, and 

 after a time this deflection settles down to a fau-ly constant 

 value, which may be considerably less than that at which 

 it started. This is an example oi polarisation ; the falling 

 off of the current has been found to be due to an opjiosing 

 electro-motive force w'hicb arises owing to the deposit of a 

 layer of oxygen on one of the platinum plates, and a layer 

 of hydrogen on the other, which layers extend into the 

 substance of the platinum as well as over the outer face. 

 The tendency which these liberated gases have to combine 

 again to form water gives rise to the opposing electro- 

 motive force which acts against that driving the current 

 through the cell, and the tinal current produced is due to 



the resultant of these two opposite electro-motive forces. 

 If the two platinum plates which have thus been exposed 

 to a cm-rent for a time be taken out of the cell and placed 

 opposite each other in a similar trough of acidulated water, 

 and then joined by a wire through a galvanometer, it will 

 be observed that they now send a current themselves, but 

 in an opposite direction to that which passed between them 

 origmaUy fi-om the external som-ce. They act as a battery 

 which becomes weaker and weaker until the layers of gas 

 sticking to their faces disappear, the oxygen and hydrogen 

 having united again to form water. Sir William Grove 

 discovered that if two plates of platinum were fixed, one in 

 a closed vessel of oxygen, and the other in one containing 

 hydrogen — the gases being obtained from any source — a 

 current passed between the two plates when they were 

 joined through a conductor ; they behaved like the plates 

 treated as above, and constituted a " gas battery." Such 

 an arrangement as that described, consisting of platinum 

 plates pre%-iously prepared by ha\ing a current passed 

 between them whilst they dip in water, is an example of 

 one of the earliest forms of secondary battery. 



Such an arrangement, however, cannot be applied to 

 any practical purpose, the duration of the second reversed 

 current being quite transitory. Gaston Plante was the 

 first experimenter to make secondary batteries on a large 

 scale. He found lead to be the most suitable metal to use 

 for this purpose. When two plates of lead are immersed 

 in dilute sulphuric acid, and a cm-rent made to pass 

 between them, one plate becomes oxidized, rusted so to 

 speak, owing to the combination with it of the oxygen set 

 fi-ee fi-om the water containing the acid. Now when the 

 cm-rent is sent in the opposite direction through the de- 

 composing solution, entering at the plate where it formerly 

 left, the plate which was previously oxidized becomes 

 deprived of its oxygen, and the other plate is oxidized 

 instead. Plante made use of this process to "form " the 

 plates to be afterwards used in his secondary batteries ; 

 cm-rents were sent first in one direction and then in the 

 other through the plates and dilute acid. In com-se of 

 time one of the plates, that which last formed the cathode, 

 or the plate where the current left the hquid, became, 

 thi-ough repeated gain and loss of oxygen, of porous 

 structm-e, exposing a large surface to the solution, whUst 

 the other became very strongly oxidized, and covered to a 

 considerable depth with lead peroxide. When these two 

 plates thus prepared ^re placed in dilute sulphuric acid, and 

 joined by a wire, a cm-rent of Electricity passes along the 

 wire in the opposite direction to that which was last sent 

 through the preparing jjlates from an external source. 

 This apparatus now forms a secondary battery or accumu- 

 lator, which is capable of furnishing a continuous flow of 

 Electricity for prolonged periods. The name " accumulator " 

 is applied to such a battery from its accumulating, as it 

 were, the chemical energy stored up in it when the charging 

 cm-rent was passed, and afterwards giving it out in the 

 form of electric energy. The battery thus laboriously 

 made by Plante, which takes months to form by repeated 

 cm-rent reversals, does not differ in any important par- 

 ticular from those in use at the present day, but now 

 they are made in a much shorter time, by starting with lead 

 oxide (or red lead) already made for one plate, instead of 

 making this by a long-continued series of altei-nately re- 

 versed cm-rents, as in Plante's original process. The plates 

 made by the Electric Power Storage Company consist of 

 gridiron-hke arrangements, with the gaps of the frame- 

 work, which is of lead, filled up w-ith little blocks of 

 minium or red lead. This has the advantage of offering 

 a large surface to the action of the liquid. Foi-merly both 

 of the plates of each cell had the plugs which filled up the 



