Page:Text-book of Electrochemistry.djvu/18

 ELECTROCHEMISTRY

CHAPTER I.

INTRODUCTION.

Fundamental Physical and Chemical Conceptions.

Polarisation. — Suppose two plates of platinum, one (P1) connected with the positive and the other (P2) with the negative pole of a galvanic battery (B), dipped into a solution (L) of sodium sulphate (Fig. 1). Two phenomena may present themselves: (a) when the electromotive force of the battery is less than about 2.2 volts, no bubbles of gas appear on the platinum plates, or (b) when the electromotive force of the battery is sufficiently great (over 2.2 volts), gas is evolved at each of the plates — oxygen at P1, and hydrogen at P2. In both cases, a current passes through the salt solution (in case (a) the current is very weak), and this can be recognised by the deflection of the needle of a galvanometer (G) interposed in the circuit indicating that the current is passing in the direction from P1 to P2. Suppose, now, that B be eliminated from the circuit by connecting P1 directly to the galvanometer by means of a wire; it will then be found that the needle of the galvanometer is deflected in the opposite direction, showing that a current is passing through the salt solution from P2 to P1, This current is due to the so-called polarisation. If whilst B