For those interested in digital lights:

How do you determine what resistor to use with which bulbs?

In series:

Resistor = __Vres_(voltage across the resistor)_
i (current in amps)

To get Vres, add up the voltage of each bulb. As an example 2 1.5V 40mA bulbs would equal 3. Subtract from the track voltage, I use 12 for my calculations.

Vres in this case would be 9. Divide 9 by .040 (40mA you don't add the mA when in series) and you get the approx. resistor needed (225 ohm). You can go up or down a little to get it the way you like it.

To find out what wattage resistor you need you just need this calculation:

P = Vres x i (current in amps)

9 x .040 = .36 watt

So you'd need a 225 ohm 1/2 (.50) watt resitor. Radio Shack sells 220 1/2 watt resistors for about 79 cents a pack. Since the resistors come in set values don't worry too much about being exact. If the calculation says 225 a 220 should be fine or you might want a 270 for a dimmer look.

Courtesy of Brian Strom