130 



THE POPULAR EDUCATOR. 



We must now explain the peculiar construction and advan- 

 tages of the several forms of battery in most common use. The 

 first employed was devised by Volta, and called by him the King 

 of Cups. A bar was made up of a rod of copper and one of 

 zinc, soldered together end to end ; this was then bent in the 

 shape of the letter u. A number of these bent rods were taken 

 and placed in a series of wine-glasses filled with dilute sul- 

 phuric acid, so that the zinc of one and the copper of the next 

 should be in the same glass. For the poles of the battery, 

 wires were fastened to plates of copper and zinc, and these 

 plates were placed in the end glasses of the series, the zinc 

 plate being in the glass which contained the copper end of the 

 rod. Fig. 6 will illustrate this arrangement. 



The first modification of this was the arrangement known as 

 Cruikshank's. A number of plates of zinc and copper are cut 

 into the form of a square with a tongue or strip projecting 

 from one side. A plate of zinc is then riveted or soldered to 

 one of copper by means of these strips, and a couple is thus 

 made of the form shown in Fig. 7. A long trough is now 

 taken and divided, by water-tight partitions, into a series of 

 cells or compartments ; and these couples are placed in them 

 so that the zinc of one couple may be in the same cell as the 

 copper of the next. Single plates of copper and zinc are placed 

 in the end cells to form the positive and negative poles, and 

 wires are connected with these. To set this battery to work, 

 we have only to fill the cells with the diluted acid. Usually 

 the couples are all connected to a rod of varnished wood, so 

 that they may be lifted together. The trough is then left filled 

 with the acid, and the plates are lowered into their places 

 when it is required to set the battery in action. By this plan 

 the battery is always ready at a moment's notice, and there 

 is no waste of the zinc from its remaining in the acid when not 

 in use. 



We may mention here that in all batteries in which zinc is 

 used, it should always be kept amalgamated, as a great saving 

 in zinc is thus effected. The mercury may, for the most part, 

 be recovered when the cell is emptied. 



The above form of battery has been still further simplified, 

 by fastening the copper and zinc plates back to back ; ttiis is 

 usually done by turning the copper plate up at the top, and then 

 soldering them together. 



The trough is then made of varnished wood, arid these com- 

 pound plates are used for the divisions. Grooves are usually 

 cut in the side, and these plates are slipped in and fastened by 

 cement. Plates of copper and zinc are, as before, placed at the 

 ends for terminals. Fig. 8 shows this battery, the thick part 

 of the plate representing the zinc, and the thin part the copper. 

 The trough in this case has merely to be filled with the exciting 

 liquid, that usually employed being dilute acid, though salt and 

 -water may be used, and a solution of sulphate of copper is said 

 to act more powerfully than either. 



Sometimes the trough is made in the shape of a second one 

 fixed to the side of the first, so that by tilting it, all the acid 

 flows into this, and leaves the plates dry. Much trouble in 

 filling the cells is thus saved. 



In practice, the battery is frequently filled with fine sand, and 

 the liquid poured over this. The action is thus more regular, 

 and there is less danger of spilling or upsetting. 



With a battery arranged in this way no greater quantity of 

 electricity is produced than from a single one of the cells, for 

 the electricity produced in each cell is employed in urging 

 forward the quantity produced in the first. The intensity, how- 

 ever, varies directly with the number. 



In the forms of this battery above described, only one side of 

 the zinc is opposite to the copper, and electricity is therefore 

 only evolved by this side. Dr. Wollaston, however, introduced 

 a modification of form, since known by his name, in which the 

 copper plate is bent round so as to enclose the zinc : small 

 pieces of wood or cork being interposed to keep the plates from 

 touching one another ; and it is found that a much greater 

 power can be obtained by using this plan of construction, which 

 ?s shown in Fig. 9, than by using those previously described. 

 Several other alterations in form have been advocated by dif- 

 ferent electricians, but as they involve no fresh principles, they 

 need not be further referred to here. We give, however, an 

 illustration (Fig. 10) of one known as Miinch's, the advantage 

 of which is, that only one trough is required, into which all the 

 couples can be introduced together. 



These batteries diminish very greatly in power after having 

 been in action for a little time. One cause of this diminution 

 is found in the fact that the acid becomes saturated with zinc, 

 and its chemical energy is therefore weakened; the other is. 

 that the hydrogen, when in its nascent state that is, when just 

 set free at the surface of the copper acts more powerfully on 

 the sulphate of zinc than it does at other times, and reduces a 

 portion of it in the form of a coating of metallic zinc on the 

 copper plate. This coating produces a secondary current in the 

 contrary direction, and thus greatly reduces the power of the 

 battery. This effect is called the polarisation of the plates, 

 and will usually pass off in a short time if the circuit be 

 interrupted. 



From these causes batteries containing two different liquids 

 are more frequently employed ; there are, however, several others 

 with only one liquid which we must describe. Ono which is a 

 good deal used in the arts is known as Srnee's battery, and is 

 represented in Fig. 11. It consists of two equal plates of ziue 

 well amalgamated ; these are kept about f of an inch apart by a 

 piece of wood, c, and are held in their place by a clamp-screw, B, 

 to which a binding screw is fixed to form the negative pole of 

 the battery. Between these there is placed a thin sheet of 

 platinised silver, which is inserted in a slit made in the wood, 

 and connected with a binding screw, A, which forms the posi- 

 tive pole. The silver is made very thin, as it does not wear, 

 and is therefore usually fixed in a slight wooden frame to pre- 

 vent its getting crumpled or torn. The reason why the silver 

 is covered with a deposit of platinum is that the hydrogen does 

 not adhere to the roughened surface thus formed, and therefore 

 the polarisation of the plate is prevented. 



A battery of this kind is very useful for a great number of 

 purposes. A "shocking machine," as it is popularly called, is 

 usually worked by one of these, and it is also found very con- 

 venient in electrotyping, as it remains in action for some time 

 without a great diminution of power. In this respect, however, 

 several other batteries are superior to it. 



A cell is usually procured into which the combination will 

 just fit, and sometimes a number of elements are connected 

 together and made to fit into one large trough. 



Walker's battery is a form of the above, tho difference being 

 that the negative plate, instead of being composed of silver, is 

 formed of graphite or gas carbon. This substance is really coal 

 distilled by heat, and is procured from the retorts used in the 

 manufacture of coal-gas. It is very hard indeed, so that it 

 requires some labour and patience to cut it into the required 

 skape. It is, however, a good conductor, especially if platinised 

 by being dipped into a solution of platinum ; and, being cheap 

 and easily procured, it is coming to be pretty generally em- 

 ployed as the negative element in galvanic batteries. The 

 power may be regulated by the depth to which it is immersed 

 in the acid. 



A form of battery more recently introduced is known as the 

 "bichromate" or "bottle" battery (Fig. 12), and is a very 

 convenient one for use on a lecture-table, or for experiments 

 requiring only a small amount of electricity. Electro-motive 

 engines, coil machines, and such pieces of apparatus may be 

 used with it very conveniently, and it is at all times ready for 

 use without further trouble. 



It consists of a plate of zinc placed between two pieces of 

 graphite, and a solution of bichromate of potash is used as the 

 exciting liquid. The zinc is fixed to a rod which slips through 

 a spring collar fixed to tho lid of the bottle, so that it can be 

 raised out of .the liquid when not in use. A binding screw, B, is 

 connected by a strip of metal with both the negative plates, and 

 another, c, is connected with the zinc, so that these screws form 

 the positive and negative poles. 



There is another kind of battery with only one liquid which 

 is coming into more general use on account of its great per- 

 manence. It is called the Marie Davy or " Persulphate " bat- 

 tery, from the exciting liquid, which consists of persulphate of 

 mercury and water. This salt is but slightly soluble ; enough is, 

 however, put in to cover the bottom of the cell to a depth of about 

 a quarter of an inch, and fresh portions of it slowly dissolve, as 

 that which was held in solution becomes decomposed. In this, 

 as in the battery last described, the elements used are zinc and 

 graphite. The latter is usually in the form of a plate, while a 

 rod of zinc, about or ^ an inch in diameter, is employed. The 

 power of this battery is not as great as that of some already 



