I've lost my memory, How do I caculate the value of this resistor?

Shoot me if I’m wrong, but don’t you need Ohm’s law for that: http://www.kpsec.freeuk.com/ohmslaw.htm?

V=IR

In a loop the current is the same for each element, so we calculate I for the smaller resistor, as we know V and R

5=I*1000
I = .005

The sum of the voltage rises equals the sum of the voltage drops in a loop, and resistors always drop voltage.

50 = v1+5
v1=45

Now we know 45 volts are used across the unknown resistor. Using the current we found previously

45 = .005*R
R = 9 k ohms

Let me know if you need it explained better.

@jackm; you can’t calculate the resistance over the first resistor, because 50V applies to the circuit as a whole; an unknown amount of voltage (0–5V, apparently) is lost over the other resistor, so there is somewhat less than 50 V over the first resistor. As shown, there is not enough information to calculate the value of the second resistor.

Your method would be correct, if you assume that the smaller resistor is the one in the loop, and that 0 to 5 V actually means 5 V exactly. Which doesn’t actually make any sense.

1–800-the-shack

@Jayne
I assumed that meant the reading went from 0 to 5 volts when the DC source was connected. I was also assumed the smaller resistor was the one that was physically smaller in the picture.. You are right though, I had to make these somewhat ambiguous assumptions to solve this circuit, I didn’t see any other way.