3-20Figure 3-14.—Ohmmeter with multiplication switch.The range used to measure any particular unknown resistance (R_{x} in view A of figure 3-14) dependson the approximate ohmic value of the unknown resistance. For instance, the ohmmeter scale of the figureis calibrated in divisions from 0 to infinity. Note that the divisions are easier to read on the right-handportion of the scale than on the left. For this reason, if R_{x} is greater than 1,000 ohms and if you are usingthe R 1 range, you will be unable to accurately read the indicated resistance. This happens because thecombined series resistance of resistors R_{x} is too large for range R 1 to allow enough battery current toflow to deflect the pointer away from infinity. You need to turn the range switch to the R 10 position toobtain the 1,000-ohm reading.Let’s assume that you have changed the range switch to the R 10 position and the pointer nowdeflects to a reading of 375 ohms, as shown in view B of figure 3-14. This would indicate to you thatunknown resistance R_{x} has 3,750 (375 times 10) ohms of resistance. The change of range caused thedeflection because resistor R 10 has only 1/10 the resistance of resistor R 1. Therefore, selecting thesmaller series resistance allowed a battery current of sufficient value to cause a readable pointerdeflection. If the R 100 range were used to measure the same 3,750 ohm resistor, the pointer woulddeflect still further to the 37.5-ohm position, as shown in view C. This increased deflection would occurbecause 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 theR 10 selection?The circuit arrangement in view A of figure 3-14 allows the same amount of current to flow throughthe moving meter coil. The same amount is allowed to flow whether the meter measures 10,000 ohms onthe 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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