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Re: Fuses



Gary Hemminger is bothered by an apparent increase in the resistance of a
fuse when more bulbs are added in parallel.

It is to be expected that the resistance of the fuse will increase as it
gets hotter and it must get hot before it melts. That's reality. But even
if the resistance of the fuse were to remain constant I would expect the
bulbs to get dimmer as more are added in parallel. The resistance of the
collection of bulbs drops each time that you add a new one so there is a
greater fraction of the total potential drop across the fuse and less
across each bulb. The same will happen even if you eliminate the fuse,
because the source has an internal resistance.
In a household circuit, the internal impedance of the source (including the
fuse) is designed to be small compared with that of any legitimate
combination of parallel loads that you attach to it.

The melting of the fuse is determined by its temperature, not simply by its
resistance and it makes sense to have a metal with a low melting point. On
the other hand you want the filaments of the light globes to reach a very
high temperature without melting. I intuit that iron is not good stuff for
making household fuses but:- Gary's demonstration sounds like a good one to
me because it raises new and interesting questions for the students.

In order to reinforce the understanding of current in circuits I like to have
kids build a circuit in which there is a homemade fuse, and
then instruct them to keep adding more bulbs in parallel. It's nice
to have a fuse that fails with some drama, but the best I've come up
with is a piece of iron from a steel wool pad. Before failure these
often glow somewhat, but they also seem to have the undesirable
property of an increasingly high resistance as more bulbs are added,
thus even with a suitable power supply rather than batteries, the
bulbs dim each time another is added and I must wave my
hands about the fact that in your living room this doesn't happen.
I'm curious to know if anyone has a suggestion for a better material,
and also, if anyone can comment on what is actually used in real
fuses. Obviously the melting point has to be right, but from what I
see in this circuit, a real fuse must also have a low
resistance.



Ian Sefton
Honorary Senior Lecturer
SUPER Group
School of Physics, A28
The University of Sydney, NSW 2006
AUSTRALIA