;iZ4 



♦ KNOWLEDGE ♦ 



[June 1, 1883. 



zinc and coppor plates, it being always carefully borne in 

 mind tliat clcnn and tliorough metallic connections are 

 absolutely essential, more especially where wc are dealing 

 with currents of low electro-motive force, such as arc gene- 

 rated by a few battery cells. The cost of a Dainell cell is, 

 as we said, small ; the outer jar and porous pot should be 

 procurabl; for a few pence. A pound of thin copper (at, 

 say, Is. 8il.). would be sufficient to make several plates 1 in. 

 by 5 in, i^inc ought not to cost Tuorc that 5d. per pound, 

 and a plat3 of j-inch metal, .")in. by 3 in., weighs a trille 

 over one pound. The terminals cost about Gd. each, but 

 vary considerably according to shape and size. Pure 

 sulphate of copper costs Gd. per pound, about ] lb. being 

 required for each cell. 



The principles involved in the Daniell cell have been 

 very extensively written and talked about ; but a brief 

 summary will not detract from any value pertaining to this 

 particular description. In the zinc division, the action 

 brings about an absorption of zinc, which is converted by 

 means of sulphuric acid into sulphate of zinc. Hydrogen 

 is at the same time released, which, entering the porous 

 pot, displaces the copper from its sulphate, forms sulphuric 

 acid, and precipitates the purest possible copper on the 

 negative or copper plate. Reduced to an equation, we get 

 Zn + SO,H^ + SO^Cu = SOjZn + SOJL-f Cu. 



Thus it will be seen that the liquid in the zinc division 

 becomes gradually a more and more concentrated solution 

 of sulphate of zinc, while the sulphate of copper solution is 

 reduced or weakened at the. same rate, and were there no 

 reserve stock of crystals in the cell, the liquid would soon 

 become colourless, and hydrogen would be deposited on the 

 copper plate. It must, however, be remembered that the 

 one great function of the bluestone solution is to prevent 

 any deposition of hydrogen, because an accumulation of 

 that gas upon the copper plate results in the rapid cessa- 

 tion of the current due to the insulation of the plate. 

 A further point to be noticed in this " polarisation " pro- 

 cess is that counter currents are started from the hydrogen 

 film to the zinc plate. It will be seen from the above 

 equation that for every atom of zinc dissolved, an atom of 

 copper is deposited, and this explains why it is that a 

 thick zinc plate is advisable, while there is practically no 

 limit to the thinness of the copper plate. In fact, batteries 

 have been constructed without any copper plate, the action 

 being relied upon to deposit a coating of copper upon a 

 sheet of lead or other conducting substance. The atomic 

 or relative weights of zinc and copper are 6-5 and 63 5 

 respectively, so that for every Q>^ grains of zinc dissolved, 

 the weight of the copper deposited will be 635 grains. 

 The molecular or relative weight of copper sulphate is 

 159-5, so that for 635 grains of copper deposited, 1595 

 grains of copper sulphate will be converted into sulphuric 

 acid. The sulphate of the zinc solution should not be 

 allowed to get too strong, otherwise a formation of in- 

 soluble zinc salts will take place. The solution, which is 

 advantageous up to a certain point, requires, therefore, to be 

 partially removed occasionally, and the cell filled up with 

 water. 



The Daniell cell has an electro-motive force of 1-079 volts 

 (see Electrical Measurement I.), the internal resistance of 

 the cell varying directly as the surface of the plates, and 

 inversely as the distance between them. The small cells 

 used by the Post Office offer 10 Ohms per cell, but the 

 resistance of such a cell as the one above described would 

 be 3 to 4 Ohms. 



As the source of a constant current, the Daniell cell is, 

 for currents of such dimensions as it is capable of main- 

 taining, as near perfection as anything human can make 



it. For this reason it is second to none where a constant 

 and steady current is required, and is extensively adopted 

 in telegraphy and electro-metallurgy. 



In consequence of the diffusion of the sulphate of copper 

 into the zinc division, a precipitation of copper at the ex- 

 pense of the zinc is always going on, and the bluestone 

 proportionately wasted. This action is represented by the 

 (!quation 



SO.Cu -f Zn = SO.Zn -|- Cu. 



The copper is deposited on the surface of the zinc, at the 

 bottom of the cell, and, generally speaking, wherever the 

 zinc touches. To prevent this as much as possible, the 

 bottom of the porous pot should be immersed for a few 

 minutes in melted paraffin wax. The upper portion of the 

 pot which stands out of the solution should be similarly 

 treated, to hinder the creeping of the solutions. Both the 

 zinc and the copper sulphates creep considerably, and, the 

 water evaporating, the sulphates crystallise. Being more 

 or less damp, they may easily short circuit the cells. When 

 any crystals are observed on the upper portions of the 

 elements, the cell should be wiped with a damp cloth. 

 It is found that the consumption goes on in an idle cell 

 almost as rapidly as in an active one, and accordingly the 

 Daniell cell is never to be recommended where only occa- 

 sional currents are required, as is the case with electric 

 bells, &c. In fact, if adopted for such purposes, the 

 Daniell cell would soon eat its head off. Nor, again, is this 

 battery advantageous where strong currents, or currents of 

 high electro-motive, are required for a short period. Other 

 forms of battery are available for such purposes, and will 

 in their turn come in for description. 



LAWS OF BRIGHTNESS. 



11. 



By Richard A. Peoctoe. 



"TTTE are next to consider the degree of brightness with 

 V V which an opaque body is illuminated, according as 

 "its distance and position vary with respect to the source of 

 light. 



It will be sufticieut to consider here only the case where 

 the illuminating body is so far off that, as seen from any 

 point of the opaque surface, it would appear very small. 



Suppose A B (Fig. 3) to be a plane opaque surface 

 squarely illuminated by a luminous surface, S S'. Let a 

 small luminous area at P emit towards the small circular 

 area C D of A B the pencil of light, PCD. Then, if the 

 area A B be shifted to the position A' B', its plane remaining 

 still square to the light rays from S S', the pencil, PCD, 

 falls upon the small circular area, C D', and obviously il- 

 luminates this area, CD', more brightly than C D in the 

 proportion that the area C D exceeds the area C D'. But 

 the area C D is to the area C D' as the square of C D 

 to the square of C D', that is, as the square of P C to 

 the square of P C. Hence the point P illuminates the 

 area A' B', more brightly than the area A B, as the square 



