DC Fundamentals Unit 13 – Direct Current Meters
Exercise 2 – The DC Ohmmeter ...............................................................................................
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to measure resistance by using a basic
meter movement. You will verify ohmmeter operation by measuring known resistor values.
EXERCISE DISCUSSION
- An ohmmeter measures resistance indirectly by measuring the current that flows through a
series circuit. - RSCALE passes 1mA of current through the meter when RCAL is at maximum resistance.
- RCAL is a rheostat that is used to calibrate the circuit and make up for decreases in battery
voltage. - The scale of an ohmmeter is opposite that of the ammeter and voltmeter scale, since an
ohmmeter scale is non-linear and read from right (0) to left (∞). - The nonlinear scale results from the Ohm’s law inverse proportional relationship between
current and resistance (with voltage held constant).