au 



KNOWLEDGE • 



[Kkh. 10, 1882. 



1>0 of little or no use for telegraphic purposPB, b>it a series 

 of cells would prcMlure a current sudioiently Ktroiif; for the 

 working of a iipcdle or Morse instrunicnt, 'I'lic first 

 liattory tlint cixnie under my notice was constructtKl as 

 follows : —In n putta-perclia trough about 2A ft. long, 

 divided into compartments, were placed altcrnote j)lates of 

 7.inc and copper. Tlie cells were lillcd with fine sand, over 

 which dilute sul|>liiiric a<id was poured. This form was 

 known as the Oruikshaiik or sand liattery ; it is now 

 entirely superseded, for it was not constant, and required 

 "refreshing" frequently, or the signals became so weak 

 that it was a very dillicult matter to read olT a message on 

 the donlile or .single needle instruments. The form of 

 liattery now adopted is called the two fluid li.attery, 

 and consists of a wooden trough, lined inside with a 

 resinous composition, wliich prevents the action of 

 the acid upon the wood. The trough is divided into 

 ten or twelve wat<r tight compartments insulated from 

 each other. In these cells stand porous earthenware pots, 

 containing a solution of sulphate of copper (blue-stone), 

 and surrounded by a semi-saturated solution of sulphate of 

 zinc. Plates of zinc and copper arc connected together by 

 a band of coj'iper, rivetted to each, and bent to allow the 

 copper plate to be in one division and the zinc in tlie next. 

 The coppers are immersed in the blue-stone solution with 

 which the porous cells are charged, the zincs in the sul- 

 phate of zinc. The last copper plate is called the positive 

 jiole of the battery, and the terminal zinc the negative 

 pole. AVhen this battery is in action, the copper of the 

 .solution is precipitated on the copper plate. If all the 

 solution were consumed, hydrogen would be deposited on 

 the copper, and the current would lose its constancy. To 

 maintain the solution in a saturated condition, and to pre- 

 vent the aoeumulation of gas bubbles, crystals of sulphate 

 of copper nre placed in the porous cells. The zinc plate is 

 consumed and the copper increased by the precipitation of 

 the metal held in solution. The sulphuric acid produced 

 by the decomposition of the sulphate permeates the porous 

 cylinder, and tends to replace the acid used up by its 

 action on the zinc. The quantity of sulphuric acid de- 

 composed in the solution of copper is regular ; the action 

 of the acid on the zinc is regular also, and thus a con- 

 stant flow of electricity is produced. Professor Thompson 

 represents the chemical action as taking place in two stages 

 Z„ + ITjROj = ZnSOj + B. 



and 



Snlptii 



rr..,lur 



Sulphalo 



iind Hydrogen. 



Zinc 

 and then — 



H2 + CuSO, = njSO, -I- c„ 



Hydrogen and ^-'^^^^^ produce Sulj^umc .„^ Copper. 



It will be seen that the zinc plate is destroyed and con- 

 verted into a sulphate, and the copper plate actually gains 

 by the action of the voltaic circuit. In the old sand 

 battery, the current was enfeebled by the accumulation of 

 hydrogen at the copper plate, and the zinc was con.sunied. 

 In the battery I have ju.st described, which is a modifica- 

 tion of the cell contrived by tlie late Professor Danicll, the 

 hydrogen bubbles are avoided and the current is constant, 

 but the consumption of zinc goes on just the same. Now, 

 the student will be able to comprehend one of the greatest 

 discoveries of modern times — that the voltaic cell is 

 reversible. " To every action there is an equal and con- 

 trary reaction." The application of Newton's words is not 

 limited to mechanical reaction — it reaches down into other 

 departments of science. In the science of electrieity and 

 magnetism, the same fundamental principle holds good. 



Dr. Sylvanus Thompson, to whom we are indebted for 

 the very best definition of the action of the voltaic battery, 

 says : — "To sep.arate an atom of zinc from one of oxygen I 



requires energy to be expended. When thus separated, 

 they have the chance of doing work in rr.-iniiliiii/, this work 

 generally appearing in the form of heat. When a piece 

 of coal is bunied — that is to say, is permitted to unite 

 chemically with oxygen its store of energy runs down and 

 manifests itself in the evolution of heat A piece of coal 

 represents a store of energy ; so does a bag of hydrogen 

 gas ; so does a jiiece of zinc, for zinc can bum directly and 

 give out heat, or may burn indirectly by being dis.solved in 

 sulphuric acid, also giving out heat A Daniell's battery 

 represents a .store of energj'. A pinch of gunpowder also 

 represents a store of energy. The amount difTers, it is 

 true, and the rate at which some of these stores can lie 

 mwle available for use also difTers widely in the different 

 cases. An ounce of coal represents an amount of energy 

 which, if entirely expended in doing work, would raise 

 fiO.*), 000 pounds one foot high again.st the force of gravity, 

 or would do 09.5,000 foot-pounds of work. In an ounce 

 of gunpowder is stored about 10,000 foot-pounds of energy. 

 An ounce of zinc represents a store of only 1 1 ."JjOOO foot- 

 pounds. An ounce of copper represents a store of about 

 09,000 foot pounds only. An ounce of hydrogen gas will 

 yield, by combining with oxygen, 2,92.5,000 foot-pounds of 

 work. Joule first showed us how to make use of facts 

 like these in calculating by its mechanical value the electric 

 power of voltaic cells. I^et us apply these considerations 

 to the storage of energy in any ordinary voltaic cell — say, 

 for example, the Daniell's cell used in telegraphy. In this 

 cell we have certain liquids containing zinc and copper 

 chemically dissolved in sulphuric acid, and into these 

 liquids dip a plate of zinc and a plate of copper. The 

 zinc plate slowly dissolves away, and, at the same time, 

 metallic copper is gradually separated out of the solution, 

 there being about 1 1-20 oz. of zinc consumed for every 

 ounce of copper deposited. Now, to separate an ounce of 

 copper from its solution in sulphuric acid, requires 69,000 

 foot-pounds of energy to be spent upon it, and as 

 1 1-20 oz. of zinc represents a storage of 118,650 foot- 

 pounds, the consumption of this weight of zinc is enough 

 to provide the 69,000 foot-pounds needed to separate the 

 copper and to leave a surplus of 49,0.50 foot-pounds. It 

 is this surplus which goe's to maintain electric currents in 

 the circuit and do electric work. But, as we have re- 

 marked, the voltaic cell is reversible. If we could take 

 such a cell and by means of some superior electro- 

 motive force drive electric currents back through the cell, 

 the whole action will be reversed. Copper will be dis- 

 solved, and zinc will be deposited. The copper in dissolving 

 will help the process by giving part of the necessary 

 energy, and our currents will thus once more give us back 

 pure zinc, and so separating out the zinc, we do work and 

 actually store energj'." To sum up, a telegraphic battery 

 is a box divided into compartments containing plates of 

 zinc and copper alternately arranged, and immersed in 

 solutions of sulphate of copper and sulphate of zinc. The 

 wires at the terminal plates are calli-d electrodes. When 

 the wires are joined together, the battery is said to be " in 

 circuit" IIow the electric current is transmitted to dis-' 

 tant places, and the instruments employed for the recording 

 of signals will be explained in a subsequent article. 



Piiiv.sF.nviNr, Fruit fou the Wixtfu.— Dry annd of all siibstanco.i 

 i.s foun(1, from llio eiporimcnt of P. Samncr, to be tlie best in which 

 to preserve fniit fur the winter. The perms of mould attnck the 

 roDf^h port inn of fniit pneked in paper, with j^rent avidity, thronph the 

 openincfB in the «ilk (lapor. If packed in straw, the least damp- 

 ness of the straw imparts a musty flavour to tlio fruit. Sand has 

 another advantajro, wliich is that the damacfed spocimons do not 

 infeot their ncifjlibours. Choose the most perfect fniit with the 

 waxy coveriuff perfect. Leaving the stalks on makes the fmiti 

 shrivel up quickly. — F.C.S. 



