Which one you use doesn't really have any major effect on simple circuits. In a series circuit, the current is constant throughout the circuit, but the voltage is shared between the resistors. Once each parallel bit has been calculated, and then the whole circuit has been done in series, the information can be put back to calculate the current or voltage in each bit of the parallel branch. Fuses are short pieces of low melting point wire. Earth-leakage detectors are designed to detect current escaping from the circuitry, and so also help prevent electrocution.
Field lines go from the north pole to the south.
The substance at the core of the field also has an effect, though it depends on the core's nature. The ampere is defined as the current which produces a force of 2 x 10 -7 N of force per meter of wire between two infinitely long wires 1 meter apart. To work out the direction, we need to remember that we are working with conventional current here, and so for a positive charge, the current will be in the direction it's moving, but a negative one will be backwards. Other than that, the right hand thing still applies. If a current is passed through, the coil will make one quarter turn, but then the force will push it back, because the current is running in the opposite sense.