168 



THE POPULAK EDUCATOE. 



some difficulty in removing any cell should it be over-filled. To 

 obviate this the zincs are much better made in three pieces, as 

 shown at B (Fig. 17). The small piece is first dropped to the 

 bottom of the cell, and then the upright pieces are put in. Very 

 great advantages, both on the score of convenience and economy, 

 are derived in practice from employing zincs of this form, for 

 the worn tops will serve well for the bottom pieces, and thus 

 there is little or no waste. If a few drops of mercury are poured 

 into each cell, it will improve the contact between the pieces of 

 zinc, and thus increase the current of electricity and at the same 

 time diminish the consumption of the zinc ; the mercury may 

 nearly all be recovered on emptying the cells. 



Within the zinc the cell c is placed. This is composed of 

 porous or unglazed earthenware, and is filled with the nitric or 

 nitro-sulphuric acid. The sheet of platinum, D, is immersed in 

 this, and is connected by a binding screw to the top of the piece 

 of zinc in the next cell. The platinum is employed very thin 

 on account of its great cost ; it is not, however, consumed at all, 

 and with care will last a very long time. When it becomes 

 much crumpled or brittle from use it may be restored by keeping 

 it for a shert time 

 exposed to a red heat, 

 and even when it gets 

 torn the pieces are 

 worth almost as much 

 as when new, for the 

 price chiefly depends 

 upon the value of the 

 metal itself. 



Fig. 19 shows the 

 way in which the cells 

 are connected together, 

 when it is required to 

 set up several. A 

 terminal binding screw 

 ia connected to the 

 end of the zinc, and 

 the platinum at the 

 other extremity is con- 

 nected to another piece 

 of zinc or other sup- 

 port, the second ter- 

 minal screw being at- 

 tached to this. With 

 fifteen or twenty cells 

 of this kind many 

 electrical experiments 

 can be shown in a 

 very effective manner, 

 and the electric light 

 can be shown, though 

 not with very great 

 brilliancy. When, how- 

 ever, sixty or seventy cells are used, a very intense and 

 powerful light can bo obtained, especially if the cells are all set 

 upon glass rods to insulate them more perfectly, and thus to 

 prevent to a great extent the local action which takes place 

 through the moisture condensed on the outside of the cells 

 forming a conductor for the current. 



It is said that a battery sufficiently powerful for trying, on a 

 small scale, most of the experiments which we shall describe 

 may be constructed at a very small cost by employing the bowls 

 of tobacco-pipes for the porous cells. The hole is stopped with 

 wax or cement, and a stand or support is made to keep them in 

 an upright position, small gallipots or glasses, such as children 

 play with, being used for the outer cells. A piece of thin zinc is 

 then bent into a cylinder, a small strip being left at one end, 

 which is bent over and has a narrow strip of platinum soldered 

 to it in such a way that it may dip into the inner cell of the 

 next element. 



In this case the platinum strip need not be larger than one 

 and-a-half inches by half an inch, and will not cost much ; all ex- 

 pense for binding screws will likewise be saved. The power of 

 this battery will be but small when compared with that produced 

 by the other cells. We have not, however, tried this ourselves, 

 and so cannot speak from experience. As a general rule, the 

 quantity of electricity depends upon the size of the negative 

 plate, and the intensity on the number of cells employed. 



When working with sixty or seventy of the larger elements 

 a considerable amount of time is occupied in filling the cells, if a 

 jug and funnel be employed, as is usually done, for if the 

 nitric acid in any case runs over into the outer cell, that must 

 be removed and emptied. The quickest and best plan, then, is 

 to place the bottle containing the acid on a shelf or ledge a few 

 feet above the level of the cells, and let the acid flow by means 

 of syphons made of good india-rubber tubing and fitted with 

 glass jets. A spring pinch-cock may be employed to check the 

 flow in passing from cell to cell, and in this way large batteries 

 may be very quickly filled. Care is, of course, required in filling 

 the syphons, and they should be well rinsed immediately after 

 use, otherwise they will be destroyed by the acid. 



The main objection brought against this kind of battery is on 

 the score of expense, the platinum being worth a great deal. A 

 modification was accordingly proposed by Professor Bunsen, in 

 which a cylinder or plate of carbon takes the place of the 

 platinum. This carbon plate may be formed from the hard 

 deposit which collects in the mouth of retorts used for the 

 distillation of coal gas ; this, however, is very hard to work, 



and powdered coke, 

 mixed with a strong 

 solution of sugar, and 

 then very tightly com- 

 pressed and baked so 

 as to form it into a 

 cake of the desired 

 shape, is often used in 

 its place. 



These batteries are 

 very generally used on 

 the Continent, as being 

 cheaper than Grove's 

 in their prime cost : 

 they are, however, less 

 convenient and rather 

 more expensive to use. 

 The difference in power 

 between the two is 

 very slight, but per- 

 haps the difference is 

 on the side of Bun- 

 sen's. Fig. 20 shows 

 the usual construction 

 of this kind of battery, 

 C being the carbon 

 cylinder, with a strip 

 of copper connected to 

 it, v the porous cell, 

 and z the zinc cylinder, 

 which also carries a 

 copper strip for con- 

 venience in joining 



with other cells. In the next illustration (Fig. 21) we see the 

 way in which a number are connected together, the positive 

 electrode being connected with the carbon pole and the nega- 

 tive with the zinc. 



From both these batteries fumes of nitrous acid gas are given 

 off copiously ; this arises from the nitric acid being reduced by 

 the hydrogen which is evolved at the negative plate. When the 

 circuit has been completed for a short time, the nitric acid in 

 the inner cell will be seen to be in a state of rapid ebullition 

 from this cause. 



As this gas is very injurious when breathed, these batteries 

 should, if possible, be set up in the open air, or, failing this, in 

 some place where there is a good draught to carry off the fumes. 

 If set in front of a good fire, this result may easily be obtained, 

 as the draught will carry the vapours up the chimney. 



Sometimes, to prevent these fumes, chromic acid or a solution 

 of bichromate of potash in dilute sulphuric acid is substituted 

 for the nitric acid; the power is, however, diminished by these 

 changes. 



Iron which has been immersed in a mixture of nitric and sul- 

 phuric acid, and thus been rendered passive, has been employed 

 by Callan instead of the carbon or platinum ; but if the nitric 

 acid becomes too weak it then acts on the iron, and causes a 

 large amount of the fumes to be given off. Platinised lead has 

 also been employed for this purpose. 



Fig. 15. 



Fig. 19. 



