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Welcome to another Lightblade learning
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lab, today we are going to use something
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that I exposed to you in the last
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session which is our K-type thermocouple
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and I’ve got some targets here, both
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paper targets and some special Acrylic
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targets made out eight millimeter
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thick acrylic that I have designed and
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made and they fit onto the same holder
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that we made several weeks ago now and
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today is going to be exciting, if I get
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it wrong
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we’re gonna have some fire if I get it
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right
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I won’t gain too much disapproval from
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Thinklaser but what I’m going to show
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you is not necessarily for you to do or
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play with
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I’m trying to demonstrate to you yet
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more technical details about this laser
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beam, because we’ve discovered over the past
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few sessions that something that should
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be maybe a four or five millimeter
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diameter CO2 laser beam who knows? It isn’t
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clearly specified but people believe
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that we’ve got a four or five millimeter
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CO2 laser beam on this 60 watt laser i’m
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afraid my answer to that is hogwash and
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I know that to be the case and I’ve
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demonstrated it to you on one occasion
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earlier you cannot burn an eight
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millimeter hole in a piece of paper with
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a four millimeter beam, you’ll burn a four
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millimetre hole okay you could say it’s
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carrying on burning outwards, but no…
Transcript For Anatomy of your CO2 Laser Beam (Cont…)
02:00
once it’s burnt out
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it’s burnt out. The energy will pass
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right through the middle of a hole and
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it won’t carry on burning out this is
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supposed to be a coherent beam of energy
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which like this pencil stays the same
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shape as it passes through the machine
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from the laser it comes out that shape and
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as it hits the last mirror it supposedly
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that shape
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let’s see what comes out of the tube and
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what eventually hits this last mirror
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because it makes a big difference to how
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you set your mirrors up and how
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accurately you set your mirrors up and
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just how much energy you’re losing
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because you haven’t set your mirrors up
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properly. We’re gonna start off by taking
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a quick look on this keyboard we’ve
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already seen that the pulse control i
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can just pulse it like that, just touch it
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and it works if i move to this control
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here and I select laser set, enter. I have
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got two modes that i can choose from one
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of them is called continuous and the
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other one is called manual this is
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rather ambiguous, manual basically means
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that i press the button pulse button
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once and I get this length of laser time
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regardless of whether i’m holding the
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button or not if I turn it over to
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continuous like that it means I get what
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I press on that button if I just go blip
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that’s all i get, so today we’re going to
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be using manual mode and these are
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milliseconds so what I’ve got there at the
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moment is half a second, i think half a
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second is going to be long enough for me
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to explore what I want to explore we
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will be changing this to 5,000 which is
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five seconds or maybe 4,000 shortly but
Transcript For Anatomy of your CO2 Laser Beam (Cont…)
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we’ll start off with 500 milliseconds
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enter we set our power to sixty-seven
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percent which is maximum
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Enter. We will just demonstrate or
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test that because we’ll put this up in
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front here and we’ll just blip the
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button and hopefully you’ll see that
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I’ve got no control
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it’s running for half a second… right
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we’ve got a round target this time
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because we’re not interested in setting
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the beam up
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what we’re interested in doing is seeing
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what the beam size is. Now i’m going to i’m
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going to put smoke in here which could
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possibly smoke the mirror now I’m not
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worried about that because the mirror is
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easily cleaned the thing that’s less
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easily cleaned and more sensitive is the
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lens in the bottom here, so i’m just gonna
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remove the lens before i start this test
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because I certainly don’t want the lens
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to get smoked if there’s any debris comes in
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there ash debris or anything it’s likely to
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go all the way down and settle on top of
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the lens. Those holes on there, those targets
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they are 3, 6, 9, 12 and 15 millimeters
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diameter so let’s just do a quick pulse
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and see what we get. 9 12-15 we’re already
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out at about six or seven millimeters and
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I’ve only given one little pulse now I’m
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purposely only doing it in pulses because
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you’ll see otherwise it will catch fire
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if I leave it running continuously so we
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do it again
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ok so we’ve got a nice ring there now
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look now I’m already out to at least 12
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millimetres and if the beam was going
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right through the middle of that hole
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it wouldn’t catch fire. Would it?
Transcript For Anatomy of your CO2 Laser Beam (Cont…)
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still smoking how can i burn a hole
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that’s 12 millimetres diameter which is
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what it is basically if we look around here
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and what you’ve got to see also is that there’s
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scorch marks around the outside he said
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ok well the scorch marks are at the top there
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because heat travels upwards
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yeah but that doesn’t account for the
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scorch marks going downwards does it so
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anything that scorching has also got heat
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on it
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so where does this beam finish it came
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out of the laser tube at twelve point
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seven millimeters diameter
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how do I know that well that’s the size
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of the aperture. Well I have to apologize
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about me and my little sketches but
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basically what happens is the laser tube
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is two mirrors, this mirror here
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allows some of the inside the laser tube
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we’re getting energy that’s bouncing
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backwards and forwards between two
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mirrors and this is the output mirror
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and it’s not hundred percent reflective
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this one is a hundred percent reflective
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and the one at this end is I don’t know
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what the percentages but let’s just say
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it’s eighty percent reflective that
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means eighty percent of the energy gets
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reflected backwards and forwards down
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here but twenty percent of the energy
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escapes out through the mirror
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ok now we’ve got a mirror that is that
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size there is a water jacket and the mirror
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sits inside the water jacket and that is
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approximately twelve point seven so one
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has to assume that the beam coming out of
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there although it’s not 12.7 diameter
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it’s a beam and what they called TEM00
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Now that M stands for mode and
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this is something called a mode 0 shaped
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beam and mode 0 shaped beam means its
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energy density is what they call
Transcript For Anatomy of your CO2 Laser Beam (Cont…)
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gaussian and basically that’s the shape
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of a bell
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so technically if I was to draw from
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here the shape of a bell
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like that, that is probably the energy
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density that’s coming out here in other
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words there’s more energy down the
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center of the beam than there is
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right at the extremity of the beam and
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so yes we have got technically here
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maybe a five-millimeter beam but what
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about all this stuff around the outside here?
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where does that go?
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it’s still energy it might not be useful
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energy as its traveling along but once
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it gets to the lens every bit of energy
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gets focussed down to something thinner
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than that little line there it is about
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two or three times the size of a human
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hair now every ounce of energy is going
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to be compressed into that little teeny
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bit their so if we throw away even ten
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percent of the energy at the extremes
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there we’ve lost ten percent of our
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energy down here it might not appear to
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be useful energy but it really is useful
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energy we want to capture the maximum we
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can so that’s why it’s most important
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that you line your beam up correctly
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when you’re setting your mirrors we
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can’t afford to lose any. In the last
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session I assumed that we had a
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10-millimeter beam and that we probably
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had about a mil and a half or so
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either side of the true centerline of
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this mirror here to catch the beam
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I’m getting a bit concerned now because
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we’ve got something there that is close
Transcript For Anatomy of your CO2 Laser Beam (Cont…)
10:00
to well certainly 12 millimetres that
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means I’ve got to be extremely accurate
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with my beam alignment. I’ve got my little water
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trough here and I much prefer to fire it
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down into water
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it’s just passing harmlessly through the
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mirrors now
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800
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900
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a 1000
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1100
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just hunting for the maximum point, it’sabout
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1100, then I gradually drag this away
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from the center you’ll see how quickly
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the power disappears see the redness
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disappearing out the probe. so i push it
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back to Center, it gets red. as I pull it out
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it goes down fairly quickly
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600, 600, 500, 400
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still at 200
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185
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push i in a little bit to see if we can make it
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change
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120
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degrees C
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so five millimetres in from each side is
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10 and that’s 21 millimetres diameter so
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that’s about an 11 millimetre beam and I’m
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still at about a hundred and ten or 120
Transcript For Anatomy of your CO2 Laser Beam (Cont…)
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degrees C now that’s above boiling point
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that’s energy that you can’t afford to
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lose
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we’ve seen how difficult it is to
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establish what size that beam is there
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we’ve also seen how relatively difficult
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it is to to measure what’s going on
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although we’ve seen it’s very high
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temperatures it would be nice to see the
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profile of this beam as i described it
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to you, the bell shape, well there is a way
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that we can see that because acrylic
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has got some fantastic properties it
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will evaporate
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proportional to the energy density or
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the amount of temperature that hits its
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surface so we should be able to burn a
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hole in there a bell shape if we’re
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lucky i’ve set the timer to five
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thousand milliseconds now, which is five
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seconds of burn time and what I’ve got
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here I’ve got a gentle air supply because
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what i want to do is make sure this
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thing doesn’t catch fire because it will
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do because the fumes that come off there
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are flammable and so what I’ve got to
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try and do is to blow the fumes away
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before they get a chance to ignite
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so let’s give it a go and see what we
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see what we get
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well there we are there’s five seconds worth of burn
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we’ve got a funny-shaped beam to start
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with the beam is not round
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it’s about seven-and-a-half nearly eight
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millimeters, along the width it’s not much
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different so it is reasonably round so if we
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take a look at the shape inside there
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yes it is a sort of a a bit of a bullet
Transcript For Anatomy of your CO2 Laser Beam (Cont…)
14:00
shape rather than a bell shape but it
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does show categorically how the energy
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density varies across the beam from very
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high in the middle where we saw that
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very high glowing temperature dropping
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off towards the side now it’s only eight
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millimeters diameter here it takes quite
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a lot of heat to evaporate and melt the
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acrylic so what sort of temperature is
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happening just outside here? another
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millimetre out here which would take it
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back to nearly 10 millimetres, well we
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can’t say because we saw that at about 10 or
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11 millimetres diameter we were seeing
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over a hundred degrees C so there’s
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still energy out here beyond this size
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but we now do the same thing coming
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straight out of the laser tube so just
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down here I’ve got a little air supply
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which is blowing air up into this area
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so we’ll run the test and see what we
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get
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you can see it’s just trying to catch
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fire
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and now we can compare
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what was at the front of the Machine and
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what was at the back of the machine
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well that’s a very interesting shape in
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there
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so you can see several things different
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about those two shapes the first thing
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is we’ve got a very very sharp-pointed
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beam here where the energy density is
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very high in the center , there’s a bit of a
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strange step there where the laser beam
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tends to change completely
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let’s just take a look at that from the
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top and you’ll see what i mean if i hold
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it in the light you can probably see
Transcript For Anatomy of your CO2 Laser Beam (Cont…)
16:00
that step in there can you see that so
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hey maybe that’s the three or four
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millimetre beam that they talk about
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and the rest of it they’re ignoring, you
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can’t ignore all this energy out here
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beyond that step you can see that step
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very clearly look
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that’s weird
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well the good news is that it’s passed
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through two mirrors and it hasn’t
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changed its size very much at all that’s
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still about 7.7 ~ 7.8 millimetres diameter
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yes it may be grown about 0.2 across
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two mirrors showing that the beam is
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maybe slightly diverging but the bigger
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tell-tale that we’ve got there is that
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we’re losing energy. Well we’re supposed
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to have about 60 watts here we decided
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that we hadn’t got that before so let’s
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just see what we got this time start
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temperature 10.2
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10.2
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minus 37.0 times two equals
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53.6
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that’s what those numbers represent 56.4
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watts here
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and 53.6 watts here
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so that’s ninety-five percent efficiency
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but across two mirrors so I’m losing two
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and a half percent per mirror
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approximately which is in the realms of
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acceptable
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it’s actually a smaller loss than I
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expected
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so the energy is still here it’s just
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not a spiky as is in this one which is
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quite fascinating
Transcript For Anatomy of your CO2 Laser Beam (Cont…)
18:02
I suppose in a strange sort of way it
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shows that we have got our mirrors set up
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quite efficiently as well, using my Machen
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meter which is what i call my Sunday instrument
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the one I only get out occasionally just to
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cross-check we got 55.2 as opposed to
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53.6 sorry we got 55.2 as opposed to
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56.4 and we got 53.6 as opposed to 53
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so using these two numbers we
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actually had ninety-six percent
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efficiency using these two numbers we had
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ninety-five percent efficiency so you
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know with this sort of instrumentation
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that’s an entirely acceptable variation
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well then thats another interesting
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observation that I’ve just done
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this one here is slightly shallower than
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that one there but this one is
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supposedly a better quality mirror with
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a higher transmission rate it’s a
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gold-plated mirror and gold is supposed
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to have a much higher transmission rate
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than molybdenum well I say much higher
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maybe one percent maybe one and a half
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percent but you know it’s supposed to be
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higher but two mirrors two gold mirrors
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as opposed to molybdenum mirrors and i
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have seen to have less power here and
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the results tend to prove that look 54
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and 50. Now I’ve started test work on my China
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machine, my other China machine using
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these gold mirrors and I’m struggling
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because I’m finding similar sorts of
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results and I was a bit puzzled because
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I was convinced that I asked for copper
Transcript For Anatomy of your CO2 Laser Beam (Cont…)
20:00
mirrors but I got gold-plated copper
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mirrors
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I don’t really want gold-plated copper
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mirrors, but I can’t actually buy copper mirrors
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and that’s the biggest problem I’ve got
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I may well have to polish the gold off of
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them to get what I want because i’m not
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so sure that it is gold, it might even be brass
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plating for all I know umm it’s yellow and
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shiny
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it certainly doesn’t perform like gold
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that’s an interesting observation that
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we found at least molybdenum mirrors are
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what they are
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so this machine is fitted with basically
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almost the best mirrors you can get.
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it’s been a fascinating session today I
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took the opportunity while I was playing
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to put some gold mirrors in and yeah
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very disappointing
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in fact i’m gonna go to the jeweler’s
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tomorrow and see if I can get the gold
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assayed, get them tested so that we can
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establish whether they are gold, they’re shiny,
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they’re yellow but they don’t perform like gold
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so i have to suspect that erm all that
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glisters is not gold
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William Shakespeare said I think it was
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him wasn’t it? Or was it my wife? perhaps in
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the next session we ought to start
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making something again rather than just
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looking at the Machine and I think
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you’ve done enough learning about the
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anatomy of the Machine the things to
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watch for the dangers I think we’ve
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covered most of the serious basics and
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technicalities of the Machine now so
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thanks to your time again next time it
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won’t be quite so boring because I hope
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we’re going to be able to do something a
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little bit more interesting and start
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thinking about craft session for you