6 



* KNOV^^LEDGK ♦ 



[July 6, 1883. 



degree the total quantity of light would thus be aftected. 

 Zollner considers that the full moon shines nearly as brightly 

 as though she were a flat disc under full solar illumination. 



(To he continued.) 



THE AMATEUE ELECTRICIAN. 



BATTERIES.— II. 



THE Bunsen cell is one which may be regarded as a 

 " typical " cell, and accordingly it is proposed to deal 

 with it in this article. It must not, however, be supposed 

 that all has been said that we intended to say on the 

 Daniell cell. There are certain modifications of it which 

 only require to be more extensively known, in order for 

 tliem to receive a much larger share of patronage. Our 

 purpose in first dealing with typical cells is to afford, sub- 

 sequently, ready means of comparison, as well as to save 

 time and space, which would otherwise be occupied with 

 repetitions which our present plan renders unnecessary. 



The Bunsen cell consists of an earthenware jar, generally 

 cylindrical, into which is placed a cylinder of zinc, made by 

 rolling a piece of flat zinc into the required cylindrical form, 

 but not allowing the two approaching edges to quite meet. 

 The object sought is to facilitate the movement of the solu- 

 tion in which the metal is immersed. Within the cylinder 

 is a round porous pot containing a rod of gas carbon. Brass 

 terminals orbinding screws are fitted on to the zinc and carbon, 

 for purposes of connection. Concentrated nitric acid (spe- 

 cific gravity 1 420) is put into the porous pot or the division 

 of the cell containing the carbon rod or plate. In the outer 

 or zinc division is poured dilute sulphuric acid, or rather 

 acidulated water. When the highest efliciency of the cell 

 is required, the solution contains one of acid to seven of 

 water, but this is a proportion not to be recommended, as 

 it causes an extravagant waste of zinc by local action, of 

 which more will be said below. A very fair proportion is 

 one of acid to twelve or fifteen of water. Hydrochloric (or 

 muriatic) acid (HCl) may be substituted for the sulphuric, 

 but it is a little more expensive. 



The action in the cell approaches, to some extent, the 

 action in a Daniell cell. The zinc is dissolved and con- 

 verted into sulphate of zinc (SOjZn), the hydrogen of the 

 sulphuric acid (SOjH^) being set free. This, entering the 

 porous pot, decomposes the nitric acid (ISTO^H), forming 

 water (OH^) and nitric peroxide (N._,Oj). Expressed chemi- 

 cally, we get 



Zn + SOiHo + 2X03H=S04Zn + 2OH2 -f NPi 

 Here it will be seen that when one volume of zinc is dis- 

 solved, two volumes of nitric acid are decomposed, and that 

 for each double volume of nitric acid decomposed, a double 

 volume of water is formed which dilutes the remaining 

 acid, a volume of nitric peroxide being also produced. 

 Thus the quantity of the nitric acid is being constantly 



the supposition of its being occasioned by the reflection of the solar 

 light." I do not know the author of the passage. I may take this 

 opportunity of noticing thac in a passage of an article on the 

 " Indications by Phenomena of Atmospheres to the Sun, Moon, 

 and Planets," in the Monthly Notices of the Astronomical Society 

 for 1862-3, page 237, Prof. Challis, while correctly stating the law 

 for the case of a self-luminous body, falls into an eiror (which the 

 above writer avoids) when speaking of opaque illuminated bodies. 

 He says "a distant spherical body, whether self-luminous or shining 

 by reflected light, would, according to the law that the intensity of 

 the radiation varies as the sine of the angle which the direction of 

 emanation makes with the surface, appear equally bright at all 

 points of the disc." Those correspondents, therefore, who have 

 addressed letters to me asking whether an opaque body under full 

 illumination should not so appear, will see that their mistake is one 

 easily fallen into. 



reduced by the decomposing action, while that which re" 

 mains is subjected to the weakening eflect of water. At 

 the same time the acid is further impaired by impregna- 

 tion with nitric peroxide, which, also getting into the pores 

 of the carbon, doubtless tends to produce a polarising 

 effect by coating the particles of carbon. A large quantity 

 of the nitric peroxide escapes into the air, and has 

 a highly deleterious eflect upon the human or any 

 other animal system. It is apparent, therefore, that 

 the Bunsen cell differs from the Daniell cell in 

 one of the most important features. In the cell last 

 mentioned, the strength of the cupric sulphate (SO4CU) 

 solution remains constant, being maintained by a reservoir 

 of bluestone crystals, but a similar reservoir of nitric acid 

 is impossible. Even were it otherwise, the production of 

 water produces a weakening effect, which can only be 

 obviated by withdrawing both acid and water. The Bunsen 

 cell is, therefore, inconstant, the strength of the current 

 diminishing with the time. Presuming the porous pot to 

 be well saturated, the diminution of the current theoreticallj' 

 commences the moment the circuit is completed, and in a 

 few minutes it may make itself perceptible. A Bunsen 

 cell, consequently, cannot be relied upon for more than 

 three or at the outside four hours. At the end of that 

 time, the nitric acid requires to be replaced by fresh acid ; 

 but the old acid may be used again twice or perhaps 

 three times if it is allowed to stand aside for awhile to 

 clarify it. 



In the outer division the zinc is rapidly dissolved, and 

 requires to be occasionally carefully amalgamated, other- 

 wise, however good the zinc may be, local action — that is 

 to say, action between two points on the zinc — will set in. 

 Wherever the metal has two different structures or mole- 

 cular arrangements, or has a chemically impure surface, 

 there will local action be seen. One has only to place an 

 unamalgamated zinc in a cell for an hour or so to see this. 

 On taking the metal out it wUl be found to be eaten away 

 very unevenly — sometimes in holes, sometimes in longi- 

 tudinal striations. 



Amalgamation is a process which homogenises the 

 structure of the metal, and covers up all superficial 

 impurities. The process may be carried out in a 

 variety of ways. The metal surface is first cleaned by 

 immersion in acidulated water. When the violent fizzing 

 that accompanies the action of the water upon a bar or 

 dirty piece of zinc has nearly subsided, the plate is coated 

 with mercury. This is sometimes accomplished by placing 

 the zinc in a solution of a mercury salt ; but the readiest 

 method is to coat the metal with the mercury either by 

 rubbing it in with a piece of tow or cotton-wool, or by 

 rolling the zinc in it. In the latter case the zinc shotild 

 be allowed to stand a short time in a dish, ifec, to drain 

 off' superfluous mercury. A pound of mercury is suffi- 

 cient to roll a six-inch cylinder in, and will last a very 

 long time. Zinc so treated infallibly works more econo- 

 mically than unamalgamated zinc, providing the entire 

 surface of the metal is so treated. The reason assigned 

 is, that the mercury associates itself with the zinc and 

 presents to the solution a surface which is absolutely 

 homogeneous, and which, therefore, does not favour local 

 action. When the metal becomes dull, reamalgamation is 

 necessary. 



The cost of making a Bunsen cell is small. One of 

 about a quart capacity is the most generally useful. The 

 outer jar would cost a few pence, as would also the porous 

 pot. The carbon rob, which may be about an inch square 

 and seven inches, or thereabouts, in length, can generally 

 be obtained at the rate of a penny an inch. The cost of 

 the carbon is almost exclusively due to the hard nature of 



