UNDOLATORY FORCES. ELECTRICITY. 



[TAMKLL'H BATTKKY. 



forms which have been suggested u convenient model 

 of obtaining voltaic electricity for experimental purpose* 

 In so doing, we shall be very brief with those which are 

 rarely used, and shall omit many, the only claim of which 

 to notice would bo their novelty, or the ignorance of 

 their inventors. 



Tho first we shall examine is the pile, which was 

 the invention of the celebrated Volta, It U easily 

 ruettxl by means of zinc and copper plates, of each 

 of which there should be about forty, of a size of two or 

 four inches square. Forty pieces of woollen cloth or 

 brown paper are required, which are to bo soaked in 

 strong salt and water, and left to drain. The pile is 

 made by building up the series in the order of zinc, 

 copper, cloth, and so on ; a wire is attached to the hut 

 plate at each end ; and so the 

 pile is completed. One of 

 these is represented in tho an- 

 nexed engraving ; in which the 

 letters or initials indicate tho 

 position of the metals and the 

 doth. (See Fig. 38). 



If the two wires, a fi, be 

 crossed, then a current of elec- 

 tricity is at once produced; 

 and if one of these points be 

 scratched over a tile, the other 

 being in contact with it, bright 

 little sparks are afforded. A 

 shock will be felt if tho hands 

 are moistened, and grasp the 

 terminal wires. 



We need not say that such 

 an arrangement is extremely inconvenient ; for it almost 

 surely happens, that the first experiment determines the 

 existence of the pile, by upsetting it altogether : if not, 

 the salt drips from each cloth, and so makes a continuous 

 conductor destroying the power of the battery. Mr. 

 Cniik.shunk improved on it by placing the plates, soldered 

 her, in a water-tight box, which was charged with 

 acid and water ; and a modification of this form of 

 battery tho cells being filled with wet sand, to prevent 

 spilling the liquid is still employed by the telegraph 

 companies at tho present day. 



The next form of battery which camo into favour was 

 thai of \Voll:i-t"n's : in which a porcelain trough, to hold 

 the acid water, was employed ; and the plates were 

 screwed into a frame. The zinc of one cell was con- 

 nected with the copper of the next, by means of a copper 

 band, and nuts and screws ; and at each end of the 

 frame a binding-screw was placed, which served to hold 

 Fig. 39. the conducting 



T T wires. One of 



^i tt ~ **\ these batteries is 



represented in 

 Fig. 39. 



We have con- 

 structed an ar- 

 rangement of a 

 similar kind ; in 

 which each plate 

 was less than the 

 twelfth of an inch 

 from each other ; 

 and forty of these 

 will enable the 

 student to try 

 numerous ex- 

 periments at little cost. A wooden trough, pitched 

 inside, and without divisions, will do to hold the acid. 



But to all these forms of batteries there is tho fatal ob- 

 jection, that the power diminishes from tho moment the 

 arrangement is put into action. This arises from a 

 variety of causes, but chiefly from tho adhesion of the 

 hydrogen to the surface of the copper plate, which as 

 effectually injures the conducting power as if it were 

 varnished over. For many years electricians combated 

 with the difficulty. Sir Humphry Davy's brilliant dis- 

 coveries were all made with the instrument we last 



described. Batteries of immense extent wen M meted, 



which, by their number of cells or tho size of the plate, 

 were intended to compensate for these deficiencies. 

 Eventually, Dr. Daniel!, of King's College, London, 

 discovered that the adhering hydrogen may be readily 

 removed by presenting the oxide of a metal, in a state of 

 solution, to it ; and by the extension of this principle, we 

 can now repeat all the usual class experiment* by a 

 battery a few inches long and broad, instead of the ex- 

 pensive and cumbersome arrangement* previously in use. 

 If a piece of common zinc or iron be dipped in sulphu- 

 ric acid and water, abundance of hydrogen is given off 

 from their surface. But on dissolving a quantity of 

 sulphate of copper (blue vitriol) in the liquid, it will be 

 found that tho gas will cease to rise, and a coating of 

 pure copper will be deposited on the metal plate. Now 

 this is an instance of what is called sccoiulary action, 

 and may be thus explained. The sulphate of cop]-r 

 contains that metal in combination with oxygon ; the 

 hydrogen, rising from the zinc, seizes the oxygen, and 

 forma water, and the metal being sot free, falls down as 

 a powder, or is deposited on the iron plate. Omitting, 

 for the present, the action which takes place on the iron 

 or zinc, tho following diagram illustrates the changes 

 which occur. 



Material*. Product*. 



Hydrogen riuir.g from j- Water. 



the zinc plate 



fOirgcm 

 Sulphate of 



copper, < Copper Copper. 



composed of 



Acid Acid. 



A Darnell's battery is constructed on the above prin- 

 ciples. It consists of an outer case of copper, a porous 

 earthenware pot, and a rod of zinc ; and one of them is 

 represented in Fig. 40 : in which C is Fig. 40. 

 the outer copper vessel ; D the porous 

 pot ; Z the zinc rod, on which is a bind- 

 ing-screw one also, B, being fitted to 

 the copper cylinder. The zinc being 

 amalgamated, is placed in the porous pot, 

 which is filled with a mixture of one 

 part of sulphuric acid to eight parts of 

 water. The copper vessel is filled with 

 the same acid mixture, which, however, 

 has been first saturated with sulphate 

 of copper; crystals of which are hung 

 on a kind of tray, T, to replenish the 

 solution with copper. If the bin<linu r 

 screws, Z and B, are connected together, 



current of electricity is generated ; 



and, after a short time, the inside of 

 the copper vessel is coated with a pink metallic deposit, 

 which is pure copper. This continues as long as the 

 battery is in action, and the strength of the copper 

 solution is kept up. Any number of these cells may be 

 joined together by means of wires inserted in the binding- 

 screws, the copper of one being connected to tho zinc of 

 the next, in the usual manner, throughout the series. 



Wo have tried many modifications of this form of 

 battery ; and, for constancy of action, have found it to 

 exceed any other. A common gallipot, a sheet of 

 copper with a wire soldered to it, and a porous pot 

 made of brown paper, with a slip of zinc in the ., 

 will answer very well for many experiments. In fact, 

 twenty such cells will enable the student to perform 

 many experiments on the small scale, and forms a cheap 

 and effective little bai terv. Dr. Danicll produced some 

 most brilliant effects with seventy cells, each twenty-one 

 in. hes high. Some years ago, we fitted up eighty flat 

 cells, each exposing a copper surface of two square I 

 which, of course, was a much larger arrangement than 

 that we have just named. Tho light between charcoal 

 points, and other effects, were of the most astonishing 

 kind. 



In using Daniell's battery, it is essential that none of 

 the copper solution should get into the pot holding the 

 zinc ; for it would immediately attack the metal, destroy 

 its amalgamation, and lower the power of the battery. 



