According to Joseph Meehan :
Jeff Klein wrote:
problem w/ running the bulbs in series, is the additional bulb, and
voltage drop, causes both bulbs to run at half strength, hence
cooler, defeating the purpose..

Unless you double the voltage, and make sure the bulbs are the same
wattage.

That's a trifle, um, unnecessary. Expending double the power, extra breaker,
somewhat code-violationish etc.

The other posting about using 6V lights in series current-matched to the
120V light is a better one. The main trick is getting 6V sockets that don't
require you to attach either side to a chassis.

Another idea is to use a much higher wattage 120V bulb as an indicator in series
on 120V. Ie: a 40W as the "heater", and 100W as the indicator. The thing
will draw something less than 40W, which should be enough to get the 100W
to glow somewhat. Pick a clear bulb to see better. Experiment...

Me? I'd probably drill holes in the enclosures, and if I felt like it,
glue some clear plastic over the holes. If the heater bulb is working,
you'll see the glow - like a pilot light.

A little more elegant would be to pick a series resistor that, when the "heater"
bulb was at full power, corresponded to about a 2-3 volt drop. Ie, on a 100W
bulb, a 5W 2 or 3 ohm resister. In parallel with the resistor, install
a LED and, appropriately sized resistor in series (a few hundred ohms I think).
Polarity of LED doesn't matter.

Heater "on", LED lit. Heater "off", LED off.

Couple bucks per "heater".
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.