ELECTRICITY. ELECTRIC CUR

RENT.—If a plate of copper and one of zinc be dipped into dilute sulphuric acid, a difference of electric potential is set up between them, the part of the zinc plate in the liquid being at a higher potential than the corresponding part of the copper plate. Now it has been found by experiment that electricity flows round a closed circuit from. a point at higher potential to one at lower potential, just as water flows from a point at higher level to a point at lower level, or heat pa es from a body at higher temp. to one at lower temp. If, therefore, the electric circuit be completed by joining the part of the copper plate outside the liquid to the corresponding part of the zinc plate by a wire or other metallic conductor, a cur- ‘rent of electricity will flow round the | circuit from the zinc plate to the copper plate within the liquid, and from copper to zinc outside. This current will continue to flow as long as the difference of potential is maintained and the circuit remains unbroken. The whole arrangement is in fact a simple voltaic cell. In practice, more elaborate cells, arranged either individually or grouped into batteries, and dynamos are used to produce currents; but however the methods employed may vary, the generation of the current depends in every case on the production of a difference of electric potential. Substances which allow the current to pa freely along them are called conductors; those which offer high resistance to the current are known as insulators. All the metals conduct well; gla , porcelain, ebonite, etc., are good insulators. The presence of the current cannot be detected by the senses. Its existence is deducted from certain properties acquired by the conducting wire: it becomes heated, a magnetic field is produced around it, and if it is severed and its ends dipped into solutions of chemical salts these latter are decomposed. Practical Units.—The practical unit of current is the ampere; the unit difference of potential or electric pre ure, the vol i. Every conductor resists the pa age of the current to some extent, and the unit of resistance is the ohm. The relation between current C, pre ure, voltage or electromotive force E, and resistance R in a circuit or conductor is given by Ohm’s Law, E=CR. If any two of these quantities are known the third is determined. Thus, if a cable is to carry 300 amperes and the potential difference between its ends is 6 volts, its resistance must be 6—300 =°02 ohms. Resistances in Series and in Parallel. When conductors are joined in series (z.e., end to end) their joint resistance equals the sum of their separate _resistances. When they are in parallel (7 e., branch out from one point and join together at another) the current divides itself among them, the smallest resistance carrying the greatest current, and the amount of current in each branch being inversely proportional to the resistance of the branch. Any number are equivalent to one resistance R. the magnitude of which may be determined from the formula —=-+-+-+ In Rom 71 fs the special case of n equal resistances in parallel, each — r, each branch carries 1 of the current and the total resistance a R-—r i