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#19
Aug1614, 01:00 PM

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We do know that 0.5 is the current through the meter movement. We do not know yet how much more current flows through R2 in parallel with the meter movement. The current through R1 and Runknown is the sum of those two currents. That's I. It's the SUM of current through the meter movement AND the current through R2. That's why you must divide the two equations to get that unknown I out of them. After you've solved for R1 and R2//Rmeter, you can go back and solve for current I with any value of Runknown. I think you'll find full scale current I is one ma, half of it through R2 and the other half through your meter movement. Halfscale current will be 0.5 ma, again half through R2 and half through your meter. Try your algebra and see if it takes you there. old jim 


#20
Aug1614, 10:25 PM

P: 92

there's something wrong in your previous expression R1=14.0*R2//Rm...
VR1=V(R2//Rm) * (R1/R1+R2//Rm)... 


#21
Aug1614, 11:19 PM

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Are you speaking of the earlier thought experiment? Same current flows through both resistances one drops 0.1 volts, the other drops 1.4 volts 1.4 = I X R1 0.1 = I X R2//Rm divide top equation by bottom equation to cancel unknown I and you get 14 = R1/(R2//Rm) R1 = 14*(R2//Rm) Show how you got your expression: R1 = 14*R1 + 14*R2///Rm 13R1 = 14*R2//Rm and a negative number for either resistance seems an unlikely result. 


#22
Aug1714, 12:16 AM

P: 92

ohh..I am so sorry... !! I told you I was so confused regarding it that I was behaving like a moron..I was nor getting even basic things... I apologise for it..you were right..!!
Now..finallyy its all clear ..I'have solved it out... R1 is coming to be 2975 & R2=50ohm... I got the points which were'nt clear ... thanks a lot for your guidance.. 


#23
Aug1714, 12:28 AM

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Do not apologize that's unnecessary . I remember very well my own stumbling up the learning curve. As you saw i still make dumb mistakes... You have been most polite and i'm happy to 'see the light come on.' That you persevered i find heartwarming. In learning we need to push ideas back and forth until our thinking leads us to math that works. Arm waving and exaggeration are useful tools. You'll be surprised how many engineers have never studied how an ohmmeter works. If you understand how your test equipment works you won't get fooled by it. And you're less apt to blow it up in a lab course. good luck in your studies. I think that with your combined curiosity and perseverance you will excel. old jim 


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