3v bulb. Stupid question??

[trastevere]

M-i-L depends on a magnifying reader torch. It takes 2x 1.5v batteries. The bulb failed and when I asked my local hardware shop for a 3v battery he told me they only have a 2.5v bulb but this would do the trick. My thinking is 2 x 1.5 = 3 therefore the filament in a 2.5 would be weaker than in a 3 and would not last for a reasonable length of time. Could someone kindly explain?

[markysngc]

2.5v will be fine, most battery's run at a little below 1.5v anyhow also if your using rechargeable ones they only give about 1.2 to 1.3v

if you did put a 3v bulb in the magnifier the bulb would be an usable brightness when the battery where brand new but would soon dim as the batterys got a little old and the voltage dropped off.

If your worried about bulb life span buy a couple while your there, they can't be more than a pound each surely

[buildersmate]

Try Maplin Electronics for the proper rating - plenty of stores up and down the country.
Agree with Mark's analysis though.

[trastevere]

Thank you both for your answers. The local hardware shop charged me 60p. By coincidence a passed a Maplins later in the day. They wanted �2.25 for the same bulb!

[ianmunt]

Mosttorch style battery units use bulbs rated below the nominal battery voltage,so it is quite normal to fit a 2.5v bulb in a two battery unit.Initially the bulb is being over run ,but the battery voltage falls fairly soon in use.If a 3v bulb was used the light would dim/become yellower in colour as the voltage dropped