simple ohmmeter is made by connecting a 4.0 V battery in series with a resistanc
ID: 2142778 • Letter: S
Question
simple ohmmeter is made by connecting a 4.0 V battery in series with a resistance R and an ammeter that reads from 0 to 1.00 mA, as shown in Figure 27-47. Resistance R is adjusted so that when the clip leads are shorted together, the meter deflects to its full-scale value of 1.00 mA. What external resistance across the leads results in a deflection of 10% of full scale? What resistance results in a deflection of 50% of full scale? What resistance results in a deflection of 90% of full scale? If the ammeter has a resistance of 22.0 Ohm and the internal resistance of the battery is negligible, what is the value of R?Explanation / Answer
Resistance R is 4V / 1 mA = 4 kohm. Deflection of the ammeter needle is proportional to current:
(a) 10%(1 mA) = 4V / (R + r) --> r = 4V / (0.1 mA) - 4 kohm = 36 kohm
(b) 50%(1 mA) = 4 V / (R + r) --> r = 4V / (0.5 mA) - 4 kohm = 4 kohm
(c) 90%(1 mA) = 4 V / (R + r) --> r = 4V / (0.9 mA) - 4 kohm = 0.444 kohm
(d) 1mA = 4 V / (R+22x10^-3) --> R + 0.022 = 4 --> R = 3.978 kohm
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