Resistance question....

[cernobila]

I have a question to do with possible resistance difference.

1, A lamp has a large brass contact area touching the battery holder. (3x18650 cells, + end)

2, Another lamp with the same specs (for the same light) has a small brass contact area touching the battery holder. All else is equal.

Would there be any difference in resistance between these two lamps and therefore a difference in performance?

I am trying to find out why the change in design, the old is with the big contact and the new is with the small contact.

 

[marksun]

Putting aside mechanical considerations, rust, etc, I think the question boils down to whether the size of a conductor affects resistance. All things being equal, cross sectional area of a conductor doesn't affect resistance but it does determine ability to carry current without getting too hot. An example of this principle is a fuse - the conductor in the fuse is purposefully small, conduction excellent, but due to its size, the fuse conductor melts when the amps rise above a certain level. A bulb contact probably has to be "big enough" to carry the current, and make good mechanical/electrical contact, beyond which there is little benefit and more material.

 

[Apollo Cree]

For low voltage applications like this, a larger contact surface doesn't necessarily mean lower resistance. A small contact area has higher pressure and will often do a better job of penetrating the oxide layer and making good metal to metal contact.

Also, since the surfaces aren't perfectly flat, the metal contacts won't touch at that many places either. A smaller surface area with higher pressure may actually end up with more area where the metal is actually touching.

If you've ever used a voltmeter, you're probably familiar with the times when a sharp probe works better than the flat side of the probe.

 

[cernobila]

The lamps in question are 12 V, 2 Amps. The older design has a larger diameter brass base that presents a large surface area to the "+" side of the battery holder. The new design has a much smaller diameter brass base that presents a small surface area to the "+" side of the battery holder........So, what has been implied in above threads may suggest that the lamp with the smaller area of contact would have slightly more output, all else being equal.......is this more or less correct?

I am trying to figure out why the change in design.......

 

[Mr Happy]

All things being equal, cross sectional area of a conductor doesn't affect resistance but it does determine ability to carry current without getting too hot.

You have this a bit wrong. The "ability to carry current without getting too hot" is the very definition of resistance. The resistance of a conductor per unit length is proportional to the cross sectional area of that conductor, and for a given current the power dissipated as heat in a conductor is exactly proportional to the resistance.

However, if a conductor is short the total resistance and total heat generated may be small compared to the whole circuit, as is the case with a fuse. The reason a fuse is enclosed in a glass or ceramic cylinder and not, say, plastic is because it may get hot in normal operation and therefore needs a heatproof enclosure.