# Homework Help: Question about Full scale deflection

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1. Dec 5, 2017

### Tony Meloni

1. The problem statement, all variables and given/known data
hello, just came across this type of question for first time.
A voltmeter with a range of 0-30volts is to be used to measure a 120 volt circuit. calculate the value of the resistor to be placed in series with the meter. the sensitivity of the meter is 1000 ohms per volt ?

2. Relevant equations

im taking a guess its 120-30 = 90 vd x 1000 = 90k ohms ? any input would help thanks
3. The attempt at a solution

2. Dec 6, 2017

### Staff: Mentor

Don't guess. What does the sensitivity tell you about the resistance at 30 V at the voltmeter?

3. Dec 6, 2017

### Tony Meloni

its telling me at 30volts its 30k ohms since its 1k per volt ..needs 90volts to drop across the resistor .. 120volt x 1000 k = 120k - 30k = 90k x .001ma = 90vd ?

Last edited: Dec 6, 2017
4. Dec 6, 2017

### Staff: Mentor

What do you multiply here and what are the units? k without a unit doesn't have a proper meaning.
That equation is wrong.
That equation is wrong as well.
That equation has at least an issue with units.

5. Dec 6, 2017

### Tony Meloni

so what are the proper steps to find the answer ?

6. Dec 7, 2017

### Staff: Mentor

If the voltmeter has 30 kOhm, what is the maximal current flowing through it?

At this current, what do you need as additional resistance to get 90 V voltage drop there?

7. Dec 7, 2017

### Tony Meloni

my choices to pick from the homework are

60k ohms 90k ohms

90ohms 150k ohms

if you take 90k x .001 you get 90vd

8. Dec 7, 2017

### Staff: Mentor

With the right units this can be useful.

Work with units. It helps spotting most errors you'll make if you don't work with them.

9. Dec 7, 2017

### Tony Meloni

what do you mean by units ?

10. Dec 7, 2017

### Staff: Mentor

Ohm, Volt, Ampere and so on.

11. Dec 8, 2017

### Tony Meloni

how would you write out the problem using ohms law ?

12. Dec 8, 2017

### Staff: Mentor

That's your homework task. You need a few simple steps in a row, going from the voltmeter to the additional resistor step by step.