Lessons in elementary physics / by Balfour Stewart.

  • Balfour Stewart
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: Lessons in elementary physics / by Balfour Stewart. Source: Wellcome Collection.

    428/502 page 408
    Fig'. 127;/. It consists of a glass jar containi;ig a rod of zinc, and a porous pot p within which is a carbon block c packed round with a mixture consisting of small pieces of carbon and pyrolusite, which is the native manganese dioxide. A saturated solution of sal ammoniac is placed in the outer glass potG ; when this has soaked through to the carbon, the cell is ready for use. The reactions taking place are as follows :— Zn Zinc + 2NHjC1 = ZnCL Ammonium _ Zinc Chloride ~ Chloride + 2NH3 + H., + Ammonia + Hydrogen zMnO., + Manganese Dioxide H.J Hydrogen — Mn.Oa Manganese Sesquioxide + + HjO Water The liberated ammonia dissolves in water. This battery is very largely used for the purpose of ringing electric bells, for which purpose it is very suitable, for although not constant in continuous closed circuit, it is capable of remain- ing in action for a considerable period if it has intervals of rest, which occurs in the ordinary use of electric bells. 393. Thermo-electric Currents— We have already alluded (Art. 290) to the current which takes place when we heat a junction of copper and bismuth. The existence of this cur- rent was first discovered by Seebeck, and it may be easily demonstrated by means of the arrangement of Fig. 94. Thus, if n s be a needle, of which n is the marked pole, and if the junction at the right be heated by a spirit-lamp, as in the figure, we shall have a current of electricity passing at the heated junction from the bismuth to the copper, its direction being denoted by the arrow-head. Hence (.-Vrt. 395) the marked pole of the needle will be deflected .as in the figure. Such a couple is called a thermo-electric couple, to dis- tinguish it from an ordinary couple, which may be termed hydro-electric. The strength or intensity of the current will of course depeiul upon the electromotive force ol the two metals which form the pile, .and we may thus draw up .a list Fig.
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