Page:Text-book of Electrochemistry.djvu/23

6 INTRODUCTION. CHAP. we differentiate between two kinds of electricity, whilst we recognise only one kind of heat (although formerly cold was often regarded as negative heat). For negative electricity the opposite to that which holds good for positive electricity obtains, i.e, negative electricity tends to pass from places at lower to those at higher potentials.

The cause of that displacement of the position of electricity which takes place without the expenditure of external work is therefore the inequality of the electric potentials at the different places. The difference of the potentials at two points is called the potential difference or electromotive force, and is that force which tends to make the electricity pass from one point to the other. In the example mentioned on p. 1 the positive electricity passes from electrode P1 to electrode P2 because the positive pole P1 has a higher potential than the negative pole P2.

Fall of Potential. — In the example quoted it is customary to speak of a fall of potential between the poles P1 and P2 in the solution.

If the potential difference between P1 and P2 amounts to V volts, and the distance between the plates is a cm., then the fall of potential is $$\frac{V}{a}$$ volts (mean value) per cm.

If, however, the cross-section of the solution is not the same throughout, then the fall of potential per centimetre will be greater where the section is smaller. In any case, the fall of potential per centimetre has a definite value at kilograms is the unit) with which, at this point, 1 coulomb of positive electricity is driven from the higher to the lower potential.

The potential corresponds, in a certain sense, to work. Thus if the potential difference between two points. P1 and