I = current strength in amperes; E = electromotive force in volts; R = resistance in ohms.
From (1) is derived the following:
E = IR (2)
R = E/I (3)
From (1) it is seen that the flow of the current is proportional to the voltage and inversely proportional to the resistance; the latter depends upon the material, length and diameter of the conductor.
Since the current will always flow along the path of least resistance; it must be so guarded that there will be no leakage. Hence, to prevent leakage, wires are insulated, that is, covered by wrapping them with cotton or silk thread or other insulating material. If the insulation be not effective, the current may leak, and so return to the source without doing its work. This is known as a short circuit.
The conductor which receives the current from the source is called the lead, and the one by which it flows back, the return.
When wires are used for both lead and return, it is called a metallic circuit: when the ground is used for the return, it is called a ground circuit. An electric current is said to be:
1. Direct, when it is of unvarying direction; 2. Alternating, when it flows rapidly to and fro in opposite directions; 3. Primary, when it comes directly from the source; 4. Secondary, when the voltage and amperage of a primary current have been changed by an induction coil; 5. Low tension, when its voltage is low; 6. High tension, when its voltage is high.