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.