One, carbon
composition vs. film resistors.
Seem to me that these are just crummy, underrated
film types.
Not really true. The manufacturing process and resistor structure
is fundamentally different. Film resistors are generally an insulating
core, with spirals of the resistive stuff (thickness, width, and material
depend on the resistance of course) around the core. Then there's a
protective coating and marking around the outside.
There are a few applications where the spiral structure has undesired
inductive effects. But then there are different varieties of metal films,
some specifically low inductance.
If you price out new carbon composition resistors, you'll know they're
different than metal films. Quarter-watt metal-films are less than a
penny each in quantity (1000 or 5000 typically). Carbon compositions
are twenty-five cents each in quantity.
I get the impression that the metal-film manufacturing process has seen
all the benefits of manufacturing technology over the past few decades,
while the carbon composition process has had no improvement for like
fifty years. Mass-produced 0.1% metal film resistors are cheaper than
10% carbon composition units.
Composition resistors also are not stable. They drift
over time.
All resistors drift over time :-). I will admit that carbon compositions
are much more drifty with respect to just about everything
(time, temperature, heat cycling, humidity, phase
of the moon, etc.)
Tim.
