Recently, my mother’s car brake light bulbs were replaced due to a failure. Afterwards, they lit so dimly that I couldn’t even see them at daytime, despite the fact that the bulbs were brand new.
I looked at the box of new bulbs that the replacements were obtained from and noticed that it said 24V21/5W. This means 24 volts, but they were supposed to be 12, and it also means that one filament is 21 watts, while the other filament is 5 watts. This is why one is thinner than the other.
Brake light bulbs contain two filaments because they are essentially two in one bulbs. One turns on when you are braking, and the other turns on at night when you turn your park and/or headlamps on so that people behind you will notice that your car is in front of them. This is a form of notice lamp. Read the caption on the image below.
In case you didn't notice (no pun intended), the reason why the driver can see the vehicle in front of them is because of the glowing tail lights. They penetrate some fog, rain, and snow so you know a car is there. All you can see is the lights, actually. Image obtained with thanks from Stefanvds(.com) on Flickr - http://www.flickr.com/photos/stefanvds/3360277443/sizes/l/in/photostream/
Most cars have a 12 volt electrical system (14 volts when the engine is on, but don’t worry about this), including this one, so I replaced the 24 volt bulbs with 12 volt ones. I found them to be much brighter and they were finally noticeable. Before replacing any of your vehicle’s light bulbs, look at the voltage printed on the lamp itself, rather than the bulbs (just in case someone put the wrong bulb in there), then remove the bulb and carry it to the store, and ask for the correct voltage.
Why the 24 Volt Bulbs Were Dimmer than the 12 Volt
The 24 volts are the same standard 21/5 watts as the 12 volt bulbs. The electrical system provided the 24 volt bulb with only 12 volts, and at 12 volts, the 24 volt bulb does not draw as much power as it should, therefore it does not light as brightly.
Simply put: At the correct voltage, the bulbs draw what they need. if it is less than adequate, they draw less current, if it is more than 12, they draw too much current.
The higher the voltage of the electrical system they are installed in, the more the bulbs draw. Putting a 12 volt bulb in a 24 volt system, however, will destroy it because it will draw excessive current.
Always find the correct bulb!