1-35Figure 1-33.—An ohmmeter with multiplication jacks.Some ohmmeters are equipped with a selector switch for selecting the multiplication scale desired,so only two test lead jacks are necessary. Other meters have a separate jack for each range, as shown infigure 1-33. The range to be used in measuring any particular unknown resistance (R_{x} in figure 1-33)depends on the approximate value of the unknown resistance. For instance, assume the ohmmeter infigure 1-33 is calibrated in divisions from 0 to 1,000. If R_{x} is greater than 1,000 ohms, and the R x 1 rangeis being used, the ohmmeter cannot measure it. This occurs because the combined series resistance ofresistor R 1 and R_{x} is too great to allow sufficient battery current to flow to deflect the pointer awayfrom infinity (). (Infinity is a quantity larger than the largest quantity you can measure.) The test leadwould have to be plugged into the next range, R 10. With this done, assume the pointer deflects toindicate 375 ohms. This would indicate that R_{x} has 375 ohms 10, or 3,750 ohms resistance. The changeof range caused the deflection because resistor R 10 has about 1/10 the resistance of resistor R 1.Thus, selecting the smaller series resistance permitted a battery current of sufficient amount to cause auseful pointer deflection. If the R 100 range were used to measure the same 3,750-ohm resistor, thepointer would deflect still further, to the 37.5-ohm position. This increased deflection would occurbecause resistor R 100 has about 1/10 the resistance of resistor R 10.The foregoing circuit arrangement allows the same amount of current to flow through the meter’smoving coil whether the meter measures 10,000 ohms on the R 10 scale, or 100,000 ohms on theR 100 scale.It always takes the same amount of current to deflect the pointer to a certain position on the scale(midscale position for example), regardless of the multiplication factor being used. Since the multiplierresistors are of different values, it is necessary to ALWAYS "zero" adjust the meter for eachmultiplication fact or selected.You should select the multiplication factor (range) that will result in the pointer coming to rest asnear as possible to themidpointofthescale. This enables you to read the resistance more accurately,because the scale readings are more easily interpreted at or near midpoint.

Integrated Publishing, Inc.

6230 Stone Rd, Unit QPort Richey, FL34668

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