To finish with I would like to show you a measurement of a very short time
involving the time taken by a pistol bullet to go between these two.
Now my students here when they see this series will think that I've chickened out
That I'm too worried to fire a pistol in front of an audience on television
It isn't that at all, one of the most famous of physicists I've mentioned
Lord Kelvin used to demonstrate an experiment in which as he
got older his sight became rather less good
[LAUGHTER]
And on one occasion he did miss the target
and it went clean through it was an elephant gun he used,
it went clean through and made a hole in the backboard
of the lecturer next door.
Lord Kelvin rushed in to see what had happened,
he found a white faced lecturer and the class promptly shouting
"Missed him, shoot him again Bill!" [LAUGHTER]
Well we will do it by getting Mr. Coates to fire the air rifle
I would like to have done this with a pistol but public safety regulations forbid it,
and so we have in fact done an experiment when no one was here
with the pistol, and we can give the answer for that too
but to demonstrate the method with the slower velocity of an air rifle pellet
I'll ask mr. Coates to fire at the target once,
We'll be very careful to try and fire on a line parallel to the axis of course,
like that.
We've let the motors rev up
[MOTORS WHIRRING]
[SHOT FIRED]
Then slow down.
And then we can trace the
bullet hole like that and you can see that there is a big lag between the
because this disk had moved all this way round while the bullet was going across
now what we have to do is to measure this angle.
it's a bit complicated because this protector was made for
the pistol which has a higher speed and therefore a smaller angle
and it only runs up to about 40 degrees so we take the mark at 30
and then we move it round to the 30 30 plus 14 so it's 44.
- 54.
- 30 plus 14, 44 so
[LAUGHTER]
and the speed we believe was, well we measured it beforehand
as two thousand seven fifty was it?
yes
twenty-two thousand five hundred centimeters a second
so dividing by roughly thirty to get to feet per second
it's... what?
[LAUGHTER]
Around just between 750 and 800 feet per second I think
If I've done the sums right
Now although this is very elementary method it is very important one because
this was a way in which the speeds of molecules was found,
one could fire molecules with a slit in the disk here
and find where they fell there.
They gave molecular speeds in this way and found
indeed that speeds of molecules were in the order of
50 thousand centimeters a second
And also if you can imagine this disc bent back on itself here
by sending light through the disc and out to a distant mirror and bringing it back,
this is in fact what Fizeau did to determine the velocity of light
you