Exercise 26.39 In the ohmmeter in the figure (Figure 1) , M is a 2.50-mA meter o

Question

Exercise 26.39

In the ohmmeter in the figure (Figure 1) , M is a 2.50-mA meter of resistance 65.0. (A 2.50-mA meter deflects full scale when the current through it is 2.50 mA.) The battery B has an emf of 1.52 V and negligible internal resistance. R is so chosen that when the terminals a and b are shorted (Rx=0), the meter reads full scale. When a and b are open (Rx=), the meter reads zero.

Part A

What is the resistance of the resistor R?

Part B

What current indicates a resistance Rx of 200?

Part C

What value of Rx corresponds to the meter deflection of 14 of full scale if the deflection is proportional to the current through the galvanometer?

Part D

What value of Rx corresponds to the meter deflection of 12 of full scale?

Part E

What value of Rx corresponds to the meter deflection of 34 of full scale?

Explanation / Answer

(a)

I =2.5 x 10-3 A

V = 1.52 V

R + 65 = V / I

= (1.52) / (2.5 x 10-3)

= 608

R = 608 - 65

= 543 ohm

(b)

if Rx = 200 ohm then total resistance will be

Rtot = (200 + 543 + 65) ohm

current = 1.52 / 808

= 0.00188

= 1.88 x 10-3 A

= 1.88 mA

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