Three lights

[jfgw]

My loft is lit by three light bulbs suspended from the ridge beam. These are controlled by three switches downstairs but I do not know which switch controls which light. I make one journey from downstairs into the loft and determine which switch is which. How do I do it?

Julian F. G. W.

[chas49]

Start with all switches off for a long time.

Switch on switch 1 and 2 for 10 minutes, then switch off number 2.

Go upstairs

Switch 1 controls the lit bulb
Switch 2 controls the unlit warm bulb
Switch 3 controls the unlit cold bulb.

[jfgw]

Quickly and correctly answered chas49.

Julian F. G. W.

[Lootman]

Switch 1 controls the lit bulb
Switch 2 controls the unlit warm bulb
Switch 3 controls the unlit cold bulb.

Presumably they are not LED lights then. LEDs do generate heat but not nearly as much as incandescent bulbs and typically have a heat sink built into them which dissipates the heat.

Compact Fluorescent bulbs don't get very hot either.

But then if the puzzle described the lights as incandescent it would be a little too easy.

[jfgw]

Switch 1 controls the lit bulb
Switch 2 controls the unlit warm bulb
Switch 3 controls the unlit cold bulb.

Presumably they are not LED lights then. LEDs do generate heat but not nearly as much as incandescent bulbs and typically have a heat sink built into them which dissipates the heat.

Compact Fluorescent bulbs don't get very hot either.

But then if the puzzle described the lights as incandescent it would be a little too easy.

CFLs definitely do get warm although not as much as filament lamps. Even LEDs give off some heat. I reckon the difference in temperature would be easily detectable by touch. Maybe there is room for experimentation.

Julian F. G. W.

[jfgw]

Presumably they are not LED lights then. LEDs do generate heat but not nearly as much as incandescent bulbs and typically have a heat sink built into them which dissipates the heat.

An experiment using a 13W Philips LED lamp (marked "100W equivalent"), Screwfix catalogue number 1616P (http://www.screwfix.com/p/philips-led-g ... tid=292403 ), fitted into an unshaded pendant lampholder suspended approximately 15 inches from the ceiling demonstrates that the method can work with LEDs. The lamp is similar in style to a GLS lamp (an old-fashioned filament "bulb") with a translucent plastic bulb and a white opaque body. Measurements were taken from both the bulb and the body using an infra-red thermometer. Unfortunately, the lamp had to be switched off to take valid measurements but the experiment clearly shows a temperature increase that persists long enough for the solution to the puzzle to work.

Warm-up phase:



The lamp was switched off and the following readings were taken:



The body still felt warm after the lamp had been switched off for 10 minutes, and the difference in temperature between the bulb and the body could easily be felt. I, therefore, conclude that chas49's answer is valid for at least the type of LED lamp tested but note that it is best to feel the temperature of the white body, not the translucent bulb of the lamp.

Julian F. G. W.

[chas49]

Presumably they are not LED lights then. LEDs do generate heat but not nearly as much as incandescent bulbs and typically have a heat sink built into them which dissipates the heat.

An experiment using a 13W Philips LED l.........

I, therefore, conclude that chas49's answer is valid for at least the type of LED lamp tested but note that it is best to feel the temperature of the white body, not the translucent bulb of the lamp.

Julian F. G. W.

I'm glad to learn I was even more right than I thought!!