inal of the battery is higher than that of its negative terminal. The difference is the electromotive force of the battery or other source. The potential at point y will be lower than the potential at point X. Thus we say that there is a potential between x and y. The potential drop is measured in volts and the magnitude of the drop depends on the resistance of the conductors and the resistor. Small-diameter pipe meter Figure A.1 A water circulating system. A.3 OHM'S LAW Battery Figure A.2 A simple electric circuit. A.2.2 Resistance In Figure A. 1, there is no practical unit to measure the "resistance" of the piping system. If the small pipe diameter is yet made smaller, the flow of water will decrease. This pipe resistance, that which reduces the water flow, is analogous to the electrical resistance of Figure A.2. The unit of electrical resistance is well defined and is called the ohm. To review these electrical units: The flow of current in a conductor is measured in amperes (A). The resistance to this flow is called the ohm (S2). The electrical pressure, called the emf, is measured in volts (V). A.3 OHM'S LAW The product of the current (amperes) and resistance (ohms) of an electrical circuit is equal to the voltage (volts). Note how we have carefully stated the units of measure. The law can be stated mathematically by E = IR, (A.1) where E is the voltage and derives from "emf," I is the current, and R is the resis- tance. Ohm's law can be stated in other ways by simple algebraic translation of terms. For example: I E/R, (A.2) R = E/I. (A.3) Example 1. If a battery has a voltage (emf) of 6 V and is connected to a lamp with a resistance of 200 S2, what current can be expected in this circuit? Use Eq. (A.2): I = 6/200 I = 0.03 A or 30 milliamperes (mA). Example 2. In a certain electric circuit we measured the current as 2 A and its resistance as 50 S2. What is the emf (voltage)? Use Eq. (A.1): E =2x50= 100 V. REVIEW OF FUNDAMENTALS OF ELECTRICITY WITH TELECOMMUNICATION APPLICATIONS A.3.1 Voltages and Resistances 