OK. The bulb does not claim to be a halogen type. It was sold as a 'bog
standard' light bulb. It looks like a standard light bulb. It is stamped
60 watts on the packet, and on the bulb itself. The glass envelope and
bulb in general, is identical in every way to what any of us would
recognise as a 'standard' incandescent light bulb given, of course, the
obvious difference between a UK bi-pad bayonet cap, and a U.S. edison
screw cap. However, it has one major difference in that instead of the
glass envelope being the size of a tennis ball, it's more like the size
of a pool ball. When installed in my bench light, which is the only
'closed in' place that I've used one so far, I did not notice any change
in light output from any other 60 watt bulb that I have used in the
light. Bear in mind that this light is used every working day to
illuminate whatever piece of kit I am working on, and has been for the
last 20 years, so I am pretty confidant that I know its 'normal'
operating characteristics.

So, if we believe the rating stamped on both the bulb itself, and its
box, and you are prepared to believe me when I tell you that with this
bulb fitted, the temperature of the shade was a whole heap hotter, then
somewhere, there must be another explanation than the two that you
believe are the only possibilities. There must be a greater degree of
heat being conducted into the base cap, in order for the temperature to
have been raised to the point where the insulation material within the
lamp itself's base, to have started to fry itself and to have destroyed
the connection pad, which is where this thread started from. There must
be considerably more heat steaming off the bulb itself, to have raised
the temperature in the upper part of the shade, to the point where the
nylon insulation around the choc bloc which was located there, has fried.
That piece of choc bloc had been there for a couple of years, and trust
me, before fitting this bulb, it was not even discoloured, let alone
crisped.

The physical contact area between the brass lamp holder, and the bracket
to which it is attached, is small, so it would seem unlikely that heat
conduction is playing much of a part in raising the temperature of the
shade. So that would leave only radiation as the mechanism for raising
the shade's temperature. I'm pretty sure that it must be a combination of
the area of the glass envelope being - what, I don't know, 30% smaller
maybe? - making for a less efficient radiator, and exacerbation of this
by that glass being nearer to the filament.

It still seems to me that this has potentially far-reaching consequences
under the right (wrong?) circumstances. The bulb was a B&Q own brand BTW.
I don't have any more in stock at the moment, but I will try to get to
the store and pick some more up, and do some further tests and
measurements.

Arfa


I've been following this thread rather loosely for the past several days,
so please pardon me if I repeat previously addressed thoughts.

Arfa, I understand that you've stated that you're using lamps labeled as
60-watt in your fixture. Have you actually measured the power consumed
(for instance, using a Kill-a-Watt or similar meter) by the lamps? It's a
possibility that the lamps have been mislabeled, and you actually have 75-
or 90-watt lamps instead of 60-watt lamps.

Since an operating incandescent lamp is essentially a resistive element,
the power consumed should be RMS power, which is the heating value of the
power. Sixty watts RMS into a resistive element should produce the same
amount of heat, whether it's in a large glass bulb or a small glass bulb.
The temperature of the bulb might be higher in a small lamp because it has
a smaller radiating surface. Power that is concentrated in a small area
will have a higher temperature than the same power that has been spread
over a larger area.

Is the radiating surface area of the new lamps small enough to cause the
increase in temperature that you're experiencing?? I suppose the math
could be done to derive the rise in temperature of your new lamps... would
be interesting to see the results.

My ending thought on the issue is that, IMHO, the telling story would be
in the reading of a power meter in the circuit. If you don't have a power
meter such a the Kill-a-Watt, then you could measure it directly with a
voltmeter and ammeter. V * I = P no need to measure phase since it's
essentially a resistor.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters
in the address)


Hi Dave
I haven't measured the actual power consumption of the new bulbs. A few
years ago, I would have been surprised if the figure quoted on a lamp was
not at least within say 5% accuracy, but these days, with the reduction in
integrity of just about everything, and the fact that much manufacturing is
now done in 'emerging' industrial countries, I would be less sure of that.
As to whether the new bulb's envelope is small enough to reduce the
dissipation of the heat by the amount noted, I'm not really a good enough
theoretical physicist to make a call of any real value, but given that this
appears to be the only mechanism by which this could be happening, I would
have to say that is probably the answer.

I don't dispute what has been said about 60 watts being 60 watts, or that
heat and temperature are not the same thing. However, radiation efficiency
is key to the relationship between the two, as we both know. A 60 watt amp
with a small silver heatsink on its output transistors will not get rid of
the unwanted heat anything like as well as if that heatsink is black, and
force air cooled.

For all I know, my bench lamp may be a 'special case', and the fitting of
the smaller bulb might just screw up the dynamics of the air circulation
within the shade. As anyone who knows the Terry's Anglepoise will agree, the
shade is not particularly well ventilated. Perhaps it would benefit from
having a ring of 5mm holes drilled around its top ... ??

Arfa
Experience: What you get when you don't get what you want