I want to power 4 1.5v incandesent bulbs with a 9v battery.I need a resistor to make sure I dont blow the bulbs.What size resistor do I need?

Depends on the nominal current that the lamps draw @ 1.5V. Also do you want the bulbs in series, adding up to 6V, or in parallel. Series is more efficient resulting in less heat and longer battery life, but if one bulb goes out they'll all go dark that way. Parallel, means four times the current draw from the battery, but one bulb out won't make them all go out.

The bulbs will be in a series.I made it so I can replace them if one goes out.

If there's a manufacturer's name and model number on the bulb we may be able to suss it out, otherwise you'd have to be handy with an ohm meter to determine the resistance of each bulb and calculate the size resister needed when using a 9v source.

They came in a bag from a train store.Only 1.5v. on the bag.Where can I find this OHM meter?

The cold resistance of a light bulb is different than when the filament is hot, so an ohm meter may not be a good choice in that regard. Better to hook a bulb up with a 1.5 V battery and a current meter to directly measure the current. Or you could simply put six bulbs in series (1.5V x 6 = 9V)to get the nine volt value of the battery and simply hide two of the bulbs somewhere. No resistor would be needed in that case.

They came in a bag from a train store.Only 1.5v. on the bag.Where can I find this OHM meter?

most multimeters will read ohms, volts, and (low) amps.

Renegade's the definite expert here: he taught me most of what I know, which is less than what he's forgotten.

Here's a multimeter. (http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100147713)

I want to power 4 1.5v incandesent bulbs with a 9v battery.I need a resistor to make sure I dont blow the bulbs.What size resistor do I need?

I'd get a resistor assortment and try it. Start with a 10K resistor and work down until the bulb does not appear dim.

20mA current is normal for an LED. 10K resistor will give you less than 1mA.

Thanks for everyones help,I went and tried a resistor and I got one bulb barely lit,so I'm trying a different approch.I hooked up the four bulbs to a 9v and got exactly what I wanted.I'm going to run this for a while and see what happen.The light is just the right look I wanted,and two bulbs will light the robot.

Thanks for everyones help,I went and tried a resistor and I got one bulb barely lit,so I'm trying a different approch.I hooked up the four bulbs to a 9v and got exactly what I wanted.I'm going to run this for a while and see what happen.The light is just the right look I wanted,and two bulbs will light the robot.

Remember, 4x1.5 volt bulbs at 9 volts in series are going to draw 1.5 times the durrent that each would do at 1.5 volts. So you're above the rated current but not ridiculously so. This will cause the bulbs to burn out sooner theoretically but probably not by much.

If you used a resistor and it was dim, go to lower impedence.

Here is a better pic of what I wanted to do.I'm only going to have it lit for a short time,so I think it will be o.k.