Page:Text-book of Electrochemistry.djvu/22

I. OHM'S LAW. 5

units into those of the "absolute system of measurement," and the electrical units have to be measured either in electro-static or electro-dynamic units. The following table shows the relationship between the values referred to: —

Practical.	Electrostatic (C.G.S.).	Electromagnetic.
$1coulomb$	$300\times 10^{7}$	$10^{-1}$
$1ampere$	$300\times 10^{7}$	$10^{-1}$
$1ohm$	${\frac {1}{900}}\times 10^{-9}$	$10^{9}$
$1volt$	${\frac {1}{3}}\times 10^{-2}$	$10^{8}$

Ohm's Law. Volt. — If the terminals of a galvanic battery of electromotive force E be connected by means of a conducting-wire so that a current passes, then the electro- motive force, the current strength (I), and the resistance (R) are connected by the following relationship: —

$E=IR$

This is termed Ohm's law, after its discoverer. That electromotive force which in a circuit of resistance 1 ohm produces a current strength of 1 ampere is taken as unit, and is called a volt Formerly, electromotive forces were referred to the tension (potential difference) between the poles of a Daniell element (zinc pole in 10 per cent. sulphuric acid, copper pole in saturated copper sulphate solution) at the ordinary temperature. This electromotive force, called a "daniell," is equal to about 1.10 volts (compare Chap. XI.). The electromotive force of a Clark element functioning at 15° is now generally used as standard (1.433 volts).

Potential. — The term "potential difference" is frequently used in place of electromotive force. Positive electricity tends always to pass from places of higher potential to those of lower potential; and this is an essential property or characteristic of the potential. In the subject of electricity the potential plays nearly the same part that the temperature does in the subject of heat, for heat always tends to pass from places of higher to those of lower temperature. In the subjects of electricity and heat, however, there is this difference, that