A simple
electrical circuit often used to demonstrate the voltage-absorbing properties of
resistors to
students in basic circuit classes. The circuit, in its simplest form, consists of a
voltage source and four resistors, connected as two
series couplets connected in
parallel.
See the diagram. (yes, those are resistors. How do
you draw resistors?)
A typical homework or test question asks the student to make the potential voltage between the middles of the two couples (v0) equal to zero volts, when, say, three of the resistors have set values, and the last is a variable resistor. If he or she remembers that voltage stays constant for circuit elements connected in parallel, and drops across each series resistor as a proportion of the individual resistance to the sum of the series' resistances (v1 = i1 * (R1)/(R1+R2)), a bit of algebra reveals that in order for v0 = 0, R1/R2 = R3/R4, or R1/R3 = R2/R4.
|------------------------------
| | |
| / /
| < <
| > R1 > R2
--- < <
| V | \ \
--- | |
| |--- + v0 - ---|
| | |
| | |
| / /
| < <
| > R3 > R4
| < <
| \ \
-------------------------------
The diagram. Many times, the bridge is drawn as a diamond, but my
ASCII drawing skills aren't quite that good.