May 9, 1872] 



NATURE 



31 



tiire-tab!c there is suspended from the top of the louvre 

 already mentioned a pendulum, which is intended to show 

 the rotation of the earth. Tlie length of it is 3S ft. ; and 

 it thus executes one complete vibration in rather less 

 than seven seconds. In showing the rotation of the earth 

 by Foucault's method, everything- depends on perfect 

 symmetry in the suspension of the pendulum. The 

 suspension used here is quite new, and, I think, will 

 prove satisfactory. I hope to take an opportunity of 

 describing it hereafter, when I can mike a statement as 

 to results obtained. 



I Ijelievc that every one who has ever had to work in a 

 laboratory with a large battery has felt how much a power- 

 ful and constant cleanly and easily managed batterv is 

 wanted. A dozen batteries have been invented, and some 

 of them patented within the last two or three years. Most 

 of them are only tit for such purposes as ringing an 

 electric bell for an instant, and even at such work as that 

 they do not last long, and not one of them is even toler- 

 ably constant, though constancy is pretended for almost 

 nil. What is wanted is a battery that will remain constant 

 for sLx inonths or a year without more attention than that 

 of occasionally wiping the outsides of the cells, and rc- 

 ])lenishing them with some salt or acid and with water ; 

 and which is powerful enough for an electric light or for 

 any other class experiment. Freedom from acid fumes is 

 also in most laboratories an essential. A great many ex- 

 periments have been made here with the view of getting 

 such a battery as I have described, and at last, as we 

 hope, with some success. 



Since the invention of Grove's battery, that and Bun- 

 sen's modification of it have been almost exclusively used 

 for the electric light. These, as far as high electromotive 

 force and smallness of resistance are concerned, are pre- 

 ferable to any other; but there are great objections to the use 

 of them in the necessity for discharging them frequently, 

 and in the emission of acid fumes, which cannot but be 

 injurious to the person who discharges the battery, and 

 which are very destructive to apparatus unless tlie battery 

 be kept in a special chamber. The battery which we are 

 employing in Glasgow, and which gives great promise 

 of being really successful, is a modification of Daniell's 

 battery. 



Sir William Thomson described (Proc. R. S., Jan. iSyr, 

 quoted in NATURE of Feb. 2, 1S71) a gravitation battery, 

 in which advantage is taken of the fact that water 

 saturated with both sulphate of zinc and sulphate of 

 copper is denser than either saturated solution of sul- 

 phate of zinc alone or sulphate of copper alone. A 

 horizontal copper plate being put at the bottom of 

 the cell and a plate of zinc near the top, the cell is 

 charged with saturated solution of sulphate of zinc, and 

 crystals of sulphate of copper are placed in a funnel, 

 whose delivering tube passes down to the bottom of the 

 cell. The superior density of the solution containing 

 sulphate of copper in addition to the sulphate of zinc, is 

 that which keeps the sulphate of copper from surround- 

 ing the zinc plate and attacking it. This arrangement, if 

 the sulphate of copper travelled towards the zinc solely 

 by diffusion, would have great advantages over any in 

 which the zinc and copper plates are placed vertically, 

 and a porous separator is used. 



It was thought that the form of cell described then 

 would turn out admirably,and it is excellent in manv ways ; 

 but it was found that a constant evolution of hydrogen 

 takes pUce at the copper plate, bubbles risin^ perpetuallv 

 from it. These cause so much stirring up of the solutions 

 that sulphate of copper is carried rapidly up to the zinc 

 plate, and both eat the zinc away, and it is itself wasted in 

 depositing copper on the zinc plate. The reason of this 

 bubbling appears to be that pirticles drop from the zinc 

 on to the copper plate, and, forming small circuits there, 

 send up hydrogen bubbles through the liquid. We have 

 now done away with the difficulty in the following way, 

 and we have got a cell which, so far as we have been able 

 to test it, promises to be in all respects satisfictory. The 

 under surface of the zinc is now covered with a sheet of 

 parchment paper (known as maniila) the edges of which 

 are brought up round the zinc, so that it is enclosed in a 

 porous cell of this material. The paper, while it does 

 not add sensibly to the resistance of the cell, acts most 

 beneficially by hindering the particles which drop off the 

 zinc from falling on to the copper plate ; an ] if there are 

 any bubbles rising from below, it prevents them from 

 bringing sulphate of copper up to the zinc plate. 



We are now employing two kinds of cells, and have forty 

 of each kind in action. The first is very similar to that de- 

 scribed by Sir W.Thomson in the paper already referred to. 

 The cell is of glass (Fig. 4) ; and this is an advantage, as the 

 condition of the solutions and metals which it contains may 

 be seen at any time. It is a circular pan* with a flat bottom 

 The diameter is 21 inches. A disc of thin sheet copper 

 is laid on the bottom ; and a thick copper wire covered 





Fig. s 



with gutta percha is soldered to the copper dis: and rises 

 to the top of the cell as an electrode. In the upper part 

 of the cell is a heavy mass of zinc, cast into the form of a 

 circular gridiron, with three ears or projection; which rest 

 on the edges of the glass. The gridiron shape is adopted 

 in order to permit the hydrogen, which we find constantly 

 being liberated, to escape. The distance between the 

 zinc and copper plates is about 2.Un. A large sheet of 

 parchment paper covers the under side of the zinc, and 

 the corners and edges of the paper are brought up round 

 the vertical sides of the zinc so as to form a kind of bacf 

 round it. The parchment paper is thus a separator be- 

 tween the mass of liquid in the cell and that immediately 

 surrounding the zinc. There is a circular hole in the 

 middle of the zinc, and the tube of a glass or earthenware 

 funnel passes through this and through a hole in the 

 parchment paper, the edges of which are tied round the 

 tube, down to the bottom of the cell. The cell is then 

 filled up with saturated solution of sulphate of zinc till the 

 level of the liquid is higher than that of the top of the 

 zinc ; and on the top of this^a layer of pure water two or 

 three inches deep is poured carefully, so as to avoid mix- 

 ing. The pure water forms an atmosphere into which 

 the sulphate of zinc formed during the action of the bat- 

 tery may diffuse, and thus crystallisation is avoided. To 

 set the cell in action crystals of sulphate of copper are 

 put into the funnel just described. The dense solution 



" Tlie p.ins th,it we are using are, I believe, mnlr- !r.. .;■',! -blowers for 

 mlllc pans. They are inexpensive, and answer - i; ■!- I., make tlie 

 bottom horizontal we cover it with santl mjistci I . i i .,<,!utijn of 



sulphate of zinc, and carefully level it bycompari , . i .m l ji f the liquid 



lying on the top. It is essentia! to have it level if >\^ \w^ii to u^e up all the 

 charge of sulphate of copper, as described in the text, to the best advantage. 



