Page:Text-book of Electrochemistry.djvu/137

 1 2 2 ELECTROLYTES. chap.

E doubled when the fall of potential per unit of length y, i.e, the

motive force, is doubled.

Now, not only the zinc ions, but also the SO4 ions, migrate

E under the influence of the fall of potential -y, but they go in

the opposite direction, namely from B to A. Hittorfs experiments, which will be considered in detail later, show that the migration velocity of the SO4 ions stands in a certain definite ratio to the velocity of the zinc ions, and this ratio is quite independent of the current strength /. It follows from this that the velocity of the sulphuric acid ions, like that of the zinc ions, is proportional to the value of the

motive force -r. This proportionality between velocity

and motive force follows from the validity of Ohm's law for solutions.

Such a law as that the velocity with which a particle moves under the influence of a certain force is proportional to this force is valid for all liquid or gaseous particles moving between other liquid or gaseous particles so long as collisions constantly take place. This law can be derived from the principles of the kinetic theory of gases, as is proved in treatises on internal friction.

We must imagine the ions as particles of a liquid which receive an acceleration under the influence of some external force, electrical or osmotic, and the velocity imparted is proportional to the force acting. The ions, like liquid particles in general, become more mobile as the temperature rises ; on the other hand, gas particles at high temperatures are more difficult to set in motion. On account of the similarity between the resistance experienced by ions in a solvent and the friction between liquid particles, the former phenomenon is called galvanic friction. This is, of course, different for different ions, and decreases with rising temperature.

Standard Units for Resistance and Electromotive

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