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Answer:

Resistance of the rheostat between ab = Rh =   1200 Ohms.

 The resistance of a rheostat will be directly proportional to the length of the rheostat.

Length of ac=    ab/4. Therefore resistance of the rheostat between terminlas ac = 

Rac =  1200/4 = 300 Ohms

Resistance of the rheostat between terminals bc = Rbc = 1200-300 = 900 Ohms

Resistance of the voltmeter connected parallel between terminals ac  =  Rv = 600 Ohms.

The effective  resistance of  Rac and  Rv ( of voltmeter) in parallel = R = Rab*Rv/ (Rab+Rv)= 

300*600/(300+600)  =  300*600/900 = 200 Ohms

Now the total resistance across the 220 Volts supply Rs will be sum of Rbc and R

that is Rs = Rbc+ R  = 900+200 = 1100 Ohms.

The voltage of 220 volts will be uniformly distributed over Rs. Therefore voltage across the voltmeter

(that is across termilals ac)  will be proportional to the effective  resistance R across ac . 

Hence , voltage across ac =   x =  (R/Rs)*220   =  (200/1100)*220 = 40 volts.    

 

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