offered by the portion of the conductor under considera- tion :
c = -
B
Joule's Law. — The electrical energy dissipated per second between two points on a conductor is expressed by the product of the current -intensity (or current-strength) by the loss of electromotive force :
h = c E, or substituting c r f or e :
h = C 2 R.
In order to find numerical expressions for c, e, and r, we must refer them to well-defined Units. These units belong to the C.G.S. system, and are so chosen that the unit of electrical energy is equivalent to ten million ergs (erg = dyne x centimetre). In other words, under the influence of unit potential, a conductor offering unit resis- tance allows a current of unit strength to pass through. In one second, therefore, a quantity of electricity is spent whose energy c e is equal to 10 7 ergs.
The theoretical definition of the units is derived from electrostatic and electromagnetic phenomena. As this definition is not of particular importance to us here, we may pass it over in silence and go on to the values of the practical units actually in use.
The unit of potential or of electromotive force is called the volt, and is so chosen that the potential of a Daniell- Fleming element is 1*072 volts.
The unit of resistance is the ohm, A column of mercury 1*063 metre long and 1 square millimetre Section at 0°C. offers a resistance of one ohm (for the legal ohm the column is only 1*060 metre, and for Siemens' s unit it is 1 metre).
The unit of cv/rrent-strength is called the amp&re, and is the strength of a current which passes along a conductor whose resistance is one ohm, under the action of an
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