OBJECT c) Calculate the total circuit resistance re- quired to limit the current to Imade with a power source of 1.5Vdc. 1. To understand the operation of an ohmmeter. 2. To construct and calibrate an ohmmeter. READ AND STUDY In your text, study the section on ohmmeters. d) Part of the total circuit resistance is the meter resistance. Calculate the additional series resistance required. INSTRUMENTS AND COMPONENTS Milliammeter 0-1mAde Electronic VOM SES 506 B1, 1.5V "C"Cell R, , ΙK Iw. R2. Potentiometer 1Kn, 2W R3, Potentiometer 10K, W R4. 1.5K2, IW Battery Holder Experiment Board Ry - Ry - Raner Rerter = NOTE: We could connect the calculated value of into the circuit, but it would cause full-scale Keries meter deflection only for exactly 1.5Vde. If the source voltage increased or decreased the meter in- dicator would not stop on the full-scale mark. It is common practice to make all or part of the series resistance variable to compensate for power source. variations. PROCEDURE The ohmmeter indicates resistance by measur- ing the current flowing through a series circuit. Although the indicator is deflected by current, the meter scale is marked off in ohms. If the voltage is held constant, by using a dry cell or other fixed source of voltage, then, by Ohm's Law, current will be inversely proportional to resistance. Most ohmmeters do, in fact, use a dry cell or battery of cells as the voltage source. Since the internal resis- tance of a cell increases as it ages, its output voltage drops under load and an "ohms" adjustment, usually a rheostat, is used to compensate for reduced out- put. During this Laboratory Experiment we will calculate the required internal resistance of an ohmmeter, then construct one, check its operation and calibrate it. O 2. a) Using dry cell B,, milliammeter, resistor R, and potentiometer R3, (wired as a rheostat), connect the circuit shown in Fig. 18-1. Connect minigator leads to the negative terminal of the cell and the bottom of potentiometer R3. These will serve as the ohmmeter probes. M. 0-imAde 1.5V O 1.a) How much voltage is required to drive the milliammeter to full-scale when in the 0-ImAde range? Vde Fig. 18-1 b) Calculate the meter resistance for the O-ImAde range. b) Set the rheostat knob to the maximum counterclockwise position. e) Clip leads A and B together and adjust the rheostat for a full-scale indication of the milli- ammeter. Is ImAde flowing in the circuit?