Figure 3-14.Ohmmeter with multiplication switch

Figure 3-13.Measuring circuit resistance with an ohmmeter.

Neets Module 16-Introduction to Test Equipment
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3-20 Figure 3-14.—Ohmmeter with multiplication switch. The range used to measure any particular unknown resistance (R_x in view A of figure 3-14) depends on the approximate ohmic value of the unknown resistance. For instance, the ohmmeter scale of the figure is calibrated in divisions from 0 to infinity. Note that the divisions are easier to read on the right-hand portion of the scale than on the left. For this reason, if R_x is greater than 1,000 ohms and if you are using the R 1 range, you will be unable to accurately read the indicated resistance. This happens because the combined series resistance of resistors R_x is too large for range R 1 to allow enough battery current to flow to deflect the pointer away from infinity. You need to turn the range switch to the R 10 position to obtain the 1,000-ohm reading. Let’s assume that you have changed the range switch to the R 10 position and the pointer now deflects to a reading of 375 ohms, as shown in view B of figure 3-14. This would indicate to you that unknown resistance R_x has 3,750 (375 times 10) ohms of resistance. The change of range caused the deflection because resistor R 10 has only 1/10 the resistance of resistor R 1. Therefore, selecting the smaller series resistance allowed a battery current of sufficient value to cause a readable pointer deflection. If the R 100 range were used to measure the same 3,750 ohm resistor, the pointer would deflect still further to the 37.5-ohm position, as shown in view C. This increased deflection would occur because resistor R 100 has only 1/10 the resistance of resistor R 10. Q-23. The R 100 resistance selection on an ohmmeter has what amount of resistance compared to the R 10 selection? The circuit arrangement in view A of figure 3-14 allows the same amount of current to flow through the moving meter coil. The same amount is allowed to flow whether the meter measures 10,000 ohms on the R 1 scale, 100,000 ohms on the R 10 scale, or 1,000,000 ohms on the R 100 scale.

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