Now I'd like to show you some experiments on resonance.
And the first one is one that was done
by John Tyndall in the middle of the 19th Century.
This is Tyndall's actual apparatus.
I don't think the glass is the same but all the rest is.
And on the top of this pipe here,
you probably can't see it but there's a very tiny flame.
Now when John Tyndall did this, he used town gas
and some time ago when we were converted to natural gas
we couldn't get it to work.
We realised that that was the reason.
So now we have to use simulated town gas.
What happens is, that if I put this tube over the top of the flame,
like that.
I think in a moment we should hear a sound.
Perhaps it wants to go just a fraction lower.
There it comes.
Now I'll just lift it up a little bit so that it's not quite so loud.
And what's happening is that hot air is rising.
And then a pulse of hot air goes up and is reflected from the back
and a pulse is, as it were, running up and down,
as the hot and cold air moves.
And also it reacts back on the gas in this pipe.
So we got quite a complicated system of things going up and down.
But it's the length of this pipe that determines the pitch of the note.
And if I lengthen the pipe, down goes the pitch.
Now if I raise it just a tiny little bit.
And stop it.
Now it stopped.
But we can start it again if I can sing
the right note that resonates with that tune.
Off it goes.
Now if I stop it.
And this time I'll sing the wrong note.
It doesn't, doesn't resonate.
Now we'll try the right note again if I can get it.
There it is.
You have to be very accurate to get that right.
And I'm going to do another of John Tyndall's experiments now.
And in order to prepare for this
I'm going to ask Bryson if he will go out.
Because he has an important part to play in this.
Not down here.
In this experiment John Tyndall wanted to show two things:
first of all resonance and secondly that sound travels along wood.
We're going to use a tuning fork
and a resonator.
This is a box which is tuned, just like the tubes in the xylophone.
If I hit the tuning fork...
I think you can hear quite clearly that the fork is amplifying the sound.
While we're going to use a much bigger fork than that.
It's about so long.
And we're going to use the resonator that matches it.
And we're going to connect the tube,
or the rod rather, a solid rod.
And if you could...
You could put it in there. If you look up the dome is going up.
And in a moment or two I think we shall see
Bryson appearing at the top there.
It'll take him a minute or two to get his safety harness on.
Because the worst thing that could happen
would be that he might come down here
and not only would we lose Bryson,
but we might lose some experiments as well.
Now he's in position, and he's going to fix the rod down there.
And now what he's going to do is to hit the big tuning fork
which you can see him holding out.
And then he's going to put the end
and I think you can hear the sound coming out here, can't you?
Could you use that once more?
I think you can clearly hear that the sound
is coming from the resonator at the bottom.
Tyndall first did this in the Christmas Lectures that he gave here in 1865
and we're fortunate that most of
the manuscripts of John Tyndall's work have been preserved
and it's so precious that we've actually asked the director himself
to take charge of the manuscripts.
And thank you very much, John,
and here we have the notes in Tyndall's own handwriting
of all the various lectures that he gave from time to time.
And beforehand, we did in fact record... Thank you very much.
We did in fact record the appropriate page about this experiment.
And Tyndall had an assistant called Barratt.
And if you look at this line here
I think you can see that it says: "Send Barratt upstairs"
Well we couldn't send Barratt, but we sent Bryson instead.
And it says: "Long rod from roof, resting upon case of tuning fork."
That's exactly what we did. And there it is.
In the manuscript of Tyndall's 1865 Children's Christmas Lectures
as they were then called.