ld due to the current flow in the circuit. This is called counter emf. This counter emf continues SO long as there is a dynamic condition, SO long as there is a change in current flow. Once equilibrium is reached and we have a current flow of exactly 2 A, the counter emf disappears. Now open the switch in Figure A.11. At the very moment of opening the switch we see an electric arc across the switch contacts. When the switch is opened there is no longer support (i.e., no current flows in the circuit) for the magnetic field in coil L. The energy stored in that field must dissipate somewhere. It dissipates back through the circuit again in the opposite direction of original current flow. Again we are faced with the fact that on opening the switch, the current does not drop to zero immediately, but takes a finite amount of time to drop to zero amperes. So a magnetic field of an electromagnet actually stores electricity. Regarding Figure A.11, we are faced with two conditions: first the buildup of current to its steady-state value, and the decay of electric current to a zero value. It may be said that an electric circuit reacts to such current changes. The magnitude of the induced emf, that reactive effect, is a function of two factors: 1. The first factor is the number of turns of wire in the coil; whether the coil has an iron core; and the properties of the iron core. 2. The second factor is the rate of change of current in the coil. These two properties only come into play when the circuit is dynamic-its electrical conditions are changing. There are two such properties that remain latent under steady- state conditions, but come into play when the current attempts to change its value. These are inductance and capacitance. For analogies, let's consider that inductance is something like inertia in a mechanical device, and capacitance is like elasticity. Inductance. The property of a circuit which we have called inductance is represented by the symbol L. Its unit of measure is the henry (H). The henry is defined as the