272 



♦ KNOV\^LEDGE *■ 



[Nov. 2, 1883. 



zinc is not transferred to the copper is to place a narrow 

 cell in a lantern and send a powerful beam of electric 

 or other light through it. Vigorous action will be seen 

 on the zinc and copper surfaces, but between the plates 

 all is quiet. There is no visible rush of hydrogen 

 bubbles through the -water. Electro-chemistry abounds with 

 illustrations of this nature. There is an e.xcellent one, 

 for which we are indebted to the ingenuity of Davy. 

 Three small glass vessels, such as watch-glasses, are 

 filled with pure water, solution of syrup of violets, 

 and sulphate of potash respectively. The centre one, 

 containing the violet solution, is connected to the 

 others by means of moistened asbestos or other threads. 

 The positive pole of a battery is placed in the water, and 

 the negative pole in the sulphate of potash. In a short 

 time the water will become very sensiljly acid. This acid 

 must have resulted from the action upon the sulphate of 

 potash. Furthermore, it cannot have passed through the 

 violet solution in a free state, otherwise the colour would 

 (in the ordinary way of chemical reaction), have been 

 changed to red. 



Bichromate liatteries, to which we now direct our 

 attention, or those in which bichromate of potash is 

 used, are a very efficient and comparatively econo- 

 mical class. They are, perhaps, more closely allied 

 to the Bunsen than to either of the otlier typical 

 batteries. Zinc in all cases constitutes the positive, and 

 carbon the negative element. The earliest forms were 

 single-fluid batteries — that is to say, they had only one 



solution in which both the plates were immersed. Fi". 1 

 represents a form known on account of its shape as the 

 " Bottle " bichromate. It consists of a long and wide- 

 necked bottle, with a brass collar, B, into which an ebonite 

 cover, E, is fitted. A brass rod, A, passing through a hole in 

 the cover carries a zinc plate, Z, which can by this arrange- 

 ment be lifted out of the solution when the current is 

 not required. The loss of zinc by local action is thus in 

 a great measure prevented. Two carbon plates, CC, are fixed 

 to the cover, one on each side of the zinc, and are connected 

 by means of a brass strap on the ebonite, so as to form 

 practically one negative plate so arranged as to call 

 into use both surfaces of the zinc plate. Z is a 

 terminal connected by means of a short brass strap with 

 the small collar through which the rod A slides. C is a ter- 



minal connected to the strap joining the two carbon plates. 

 Small insulating studs attached to the zinc prevent its 

 making contact with the carbons, an eventuality which 

 would obviously short-circuit the cell. The solution consists 

 of bichromate of potash (KoCr.Oy) and water, acidulated 

 with sulphuric acid (H.SOj). The solution should be a 

 saturated one — that is, it should contain as much bi- 

 chromate as it possibly can. It is best in preparing the 

 solution to apply heat, as the water then takes up an in- 

 creased proportion of the salt. A pint of water will 

 dissolve about three ounces of bichromate. When cool, 

 four ounces of sulphuric acid should be added, and when 

 the current becomes weak a further addition of four 

 ounces of the acid will restore it. This latter remark, 

 however, applies only to single fluid cells. 



The action is somewhat complicated, and is furthermore 

 accompanied by energetic local action. The zinc acting 

 with sulphuric acid forms the customary sulphate of zinc 

 (ZnSOj), hydrogen being released. The bichromate of 

 potash acting with other portions of sulphuric acid pro- 

 duces chrome alum (2KC12SO4), water (OH.,), and free 

 oxygen. The free or nascent hydrogen combines with the 

 free oxygen and forms wa*er, thereby preventing to some 

 extent the polarisation of the plates. The action may be 

 represented by equations thus — 



3Zn-f3H,SO,=3ZnS04-f.3m I qnTT 



K,Cr,0-+4H,SO,=2KCr2S6, + 40H.,-f 30 / "^^^- 

 Or putting the two actions into one line we get- — 

 3Zn + 7H.,S04 + K„Cr,0; = 3ZnS0j + 2 KOr2S04 -F 70H, 



It will be seen by these equations that for every three 

 atoms of zinc dissolved, and 7 molecules of sulphuric acid 

 reduced, only 1 molecule of bichromate is aflfected. This 

 is fortunate, on account of the comparatively weak solu- 

 bility of the salt. The chrome alum forms with water, 

 crystals of a violet tinge and of great beauty. They are, 

 however, unwelcome guests, as they form chiefly upon the 

 carbons and the containing cell, and considerably impair 

 the cell's efficiency. Their formation, too, proceeds when 

 the battery is lying idle. One of the prettiest objects 

 throughout the range of battery products is a carbon plate 

 covered with chrome alum crystals. When held up to the 

 light, so as to allow the rays to pass through them, their 

 beauty becomes at once apparent. 



Very frequently the cells are made with several plates 

 of carbon and zinc alternating, the carbons being con- 

 nected to one terminal, and the zincs to another. This, of 

 course, is equivalent to employing a large plate of zinc 

 opposed to a large one of carbon. More of this, however, 

 in our next article. 



The battery is for some purposes very useful. It yields 

 an electromotive force of 2 to 2 -12 volts, and is therefore 

 in this respect more powerful than the Bunsen or Grove 

 cells. Its resistance, too, is naturally very low, being 

 but the fraction of an Ohm. It, however, very speedily 

 loses its power, and the current dwindles away to nothing. 

 Raising and lowering the zinc once or twice overcomes this 

 defect for a time, and in experiments we have been able to 

 obtain satisfactory currents, by adopting this device, for 

 hours together, while one charging has sufficed to supply a 

 current three hours a day, twice or thrice a week, for six 

 months or more. It is apparent, however, that the 

 longevity of a cell is limited by the consumption of its 

 constituents. The bichromate cell is, nevertheless, very ser- 

 viceable where strong currents are required for short inter- 

 vals, and where the nitrous fumes arising from the Grove 

 and Bunsen are objectionable. Upon this point we shall 

 have a few words to say when speaking of the practical ap- 

 plication of the bichromate in the form of the Fuller cell, of 

 which, by-the-way, several thousands are at present in usa 



