Short Time Intervals (1960)

today I'd like to talk to you about short intervals of time intervals shorter than a second and some of the way is that we can measure them now there are a number of ways of getting into this subject but today I'd like to take one specific event something that we're all fairly familiar with and something that we think of as happening fairly quickly and see how accurately we can time it the event that I'd like to study is lightning the average lightning flash lasts for about a half a second and it appears to the eye to flicker a little bit if you were very fast you could probably measure the total duration of The Flash with a stopwatch but let me ask you this does The Flash start from the cloud and go down to the ground or does it start from the ground and go up to the cloud and that flickering what's causing that is it because one single stroke is varying in brightness or because there are a number of Strokes one coming right after the other you probably don't know the answer to these questions because the eye just doesn't respond to many things that happen very quickly but in physics very often we have to know what happens in very short intervals of time intervals as short as a millionth of a second so we have to develop special techniques for studying them the first technique I'd like to talk about today is a photographic technique about 100 years ago photographers were making still pictures of lightning by time exposure most of the pictures as you would expect came out looking like this but then her picture was taken by this same method which came out looking something like this at first some thought a strong wind had blown the lightning sideways then they began to experiment a little and they found that pictures like these always resulted when the camera was knocked sideways when the lightning struck in order to understand what this picture of lightning shows we must first realize the significance of the fact that the film was moving sideways while the picture was being taken to make this clear Let's do an experiment when I push this switch the battery causes the pen in this recorder to move and Mark this piece of paper I'll start it the mark shows that I have closed the switch but it doesn't show for how long and if I close and open the switch more than once I cannot determine the number of times that I've done it from this record but by doing one simple thing by making the paper move I can put much more information on the record the length of the Mark is now a measure of the time I closed the switch and it is immediately obvious how many times I've done this in order to measure these intervals I'll have to know what speed the paper is moving at this timer is quite accurate I'll hook it up to the same switch so that it starts and stops along with the PIN that was 2 and 1/2 CM on the recorder and 5 seconds on the timer so when the paper runs at this Speed 2 1/2 CM about an inch equals 5 Seconds now I trust this timer for a number of reasons every time I do the experiment the recorder and the timer agree and the recorder agrees with my watch and it agrees with my own sense of how long 5 Seconds is but if I want to measure events that happen much more rapidly I can no longer judge from my own sense of time whether the recorder and the timer are right all I can do is speed up the recorder by a known amount when the paper moves 25 times faster 1 cm should correspond to 8 hundredths of a second this Mark is a little less than a centimeter in indicating a period of about 7 hundredths of a second this is also how long the timer indicates the switch was closed so we see this technique Works in measuring shorter intervals if I move a piece of film rather than a paper I should be able to perform the same kind of experiment except this time I will Mark with a light source rather than with a pen this experiment will take us right back to our pictures of lightning here are my light sources a fluorescent light connected up to the 60 Cycle power lines and a candle I'll light it if we take a still picture of these two we get something like this very much as the eye sees it now I'll take a picture with the camera moving the camera is mounted on this phonograph turntable with all this weight on it the table will turn at a little less than 30 Revolutions a minute the circumference of the table is 60 in so when the motor is running the film and the camera will turn in this direction at a speed of 1 in in a 30th of a second I press the shutter when the camera is pointing in this direction the picture will be taken and the shutter will close when the camera is over here the candle which was on the bottom left a smooth steady smear across the picture the flues bulb on the other hand has made a series of spaced lines now if I measure these lines I can tell that in a 30th of a second the time it takes the film to move a little less than an inch the light goes on four separate times in one full second then the bulb would be blinking 120 times now here's that picture of lightning again this is no longer an enlargement but is the size of the original negative in the camera that took this picture the film was moved in this direction at a speed of about 2 in per second if a stroke of lightning were a single flash this stroke would make a solid blur across the film the way the candle did but this picture looks much more like the one of the fluorescent bulb in this flash it looks like there were 20 22 separate Strokes if we measure for example the distance between the fourth and the fifth stroke we find that it's a 16th of an inch the film moves 2 in a second so we know these Strokes occurred 132 of a second apart in a sense this sort of picture is a graph in which time is plotted as a distance but for all we know each one of these Strokes May consist of many more Strokes the most obvious way to look for more detail is to move the film faster now there are many types of cameras which do this one of them is in operation at the New Mexico Institute of Mining and Technology at sakuro New Mexico this camera moves the film past the lens at a speed of 12 in per second the film which is about 8 in wide is held on two spools in this Carriage which can be revolved at a constant speed the film comes off one SP spool is held flat behind this plate and goes onto the other spool after each exposure a the film can be Advanced behind the lens by turning the spools if you held a pencil against the film at the point where the lens is it would draw a circle on the film as it turned for the same reason exposures taken through the lens come out as circular pictures the camera in its housing is mounted in this shelter on top of the research building the installation can be maneuvered to cover any part of the Horizon the camera on the right is the so-called slow speed camera which tracks back and forth at 2 in per second the faster moving camera is simultaneously taking pictures of the same lightning strokes this photograph is an enlargement of a picture taken in New Mexico with the slow-moving camera this is a picture of the same stroke of lightning taken with a faster moving camera the circular piece of film has been cut into five sections let's look at stroke 24 on this slow moving film it appears as a single flash where the film is moving 10 times as fast it now appears as two separate parts the film moves in this direction so we know that the faint stroke on your right came first coming from the cloud down to the ground followed by the brighter return stroke which goes from the ground up to the cloud by knowing the speed that the film was moving and by measuring the photograph we can show that The Faint stroke started down from the cloud a thousandth of a second before the brighter return stroke got back up to the cloud as we continue to measure shorter and shorter times it becomes more and more difficult to refer the times the way that we have 9 10 thousand of a second for example so let's do this let's call a thousandth of a second a millisecond a millionth of a second a microsecond and a thousandth of a millionth of a second a mill microsecond now in our photographic techniques measured events that occur in this time range now let's study things that happen a thousand times faster to do this we're going to use the oscilloscope a very common research tool this tube is similar in many ways to the one in your television set and it's similar to the one in the aill scope as well back here a beam of electron starts out and Strikes the face of the tube where it hits it causes the chemical to Glow so that we can follow the motion of the beam if I connect the positive lead of the battery to this terminal the beam moves over to your right if I connect the positive lead over on the left hand side the beam moves to the left on top moves up and on the bottom moves down I can do the same thing with my oscilloscope up down down left right we're going to use the oscilloscope to measure short time intervals in our camera experiment we moved the film and we kept the light source still in the oscilloscope we have to do it the other way around we keep the film this screen still and we move the light source the beam of electrons I need to make the spot move at a known uniform speed across the tube face this movement is called The Sweep because the spot sweeps uniformly across the screen I can vary The Sweep from once every 50 seconds all the way to once every 10 microc seconds at faster sweeps it looks just like a line now I'll set the sweep for one second and do the same sort of experiment we did with the pen recorder instead of a switch I will use a flash light shining on this photo cell when I block the light to the photo cell I do the same sort of thing as opening and closing a switch this causes the oscilloscope beam to move up and down the length of the deflection is a measure of the length of time my hand was in front of the photo cell if I move my hand in front of this twice very quickly we can see immediately that two separate events occurred now to show you how the oscilloscope can measure very rapid events I will try interrupting the light thousands of times faster I'm going to use this motor and gear the gear has 140 teeth and turns at 60 Revolutions a second I'll position the photo cell so that the light passing through the small hole falls on it when the wheel turns the teeth momentarily block off the light as you can see on the scope now I'll start the motor and adjust the sweep speed so that the beam crosses the grid once every millisecond now I'll count the number of interruptions from here to here is 1 2 3 4 5 6 7 8 and about a fifth that's 8.2 interruptions in a millisecond or 8200 per second now this motor runs at 60 Revolutions a second and there are 140 teeth on the gear so this means that the light is being interrupted 8,400 times a second when we made the experiment on the oscilloscope we got 8,200 interruptions so the scope is measuring correctly to about 2% if I use a higher sweep speed I can measure down to micros seconds with this scope the oscilloscope is used to examine fine details of lightning Strokes when used with this sensitive photocell unit this cell responds to minute very rapid changes in the intensity of the light in lightning Strokes the photo cell is here mounted at the top of this observation tower at the New Mexico Institute it is connected to an oscilloscope in a room within the tower you see two lines on the scope face One records the changes in the output of the photo cell the other records electrical changes in the atmosphere this viewer makes it possible for an observer to watch the oscilloscope while at the same time a 35mm camera makes a constant photographic record of the patterns on the tube face the photographic records of the oscilloscope patterns are cataloged and studied in detail each of these lines is a photograph of One Sweep across the oscilloscope face these lines can be drawn end to endend and scaled up in size for more careful study in this chart we see Strokes four and five of one lightning flash they were separated by 40 1 milliseconds if we look closely at stroke 5 we see that it lasted from here to here 2.4 milliseconds you can see clearly where the return stroke started if we look at stroke four we immediately see how it differed from stroke 5 here the leader stroke lasted longer and was not as brilliant the return stroke was more abrupt also the total stroke lasted 4.5 milliseconds in this oscilloscope record we can see details of the various parts of the lightning stroke details in light fluctuations only 10 micros in duration lightning is an exceedingly complicated electrical discharge we have by no means shown all the techniques used to measure it but the purpose of this film wasn't to give you the last word on lightning what we wanted to do was to look at something that happened too rapidly for us to be aware of directly and show you some of the techniques and equipment used to measure it equipment such as a pen recorder a highspeed [Music] camera and an oscilloscope there are other techniques for measuring short time intervals and observing events of short duration for example this simple stroboscope with this sort of tool you can take a long careful look at recurring events like the Turning of a fan blade and there are exceedingly high-speed Motion Pictures which when projected slow down action for analysis there is one thing that all these techniques share in common they all extend our awareness of the world around us past the limitation of our senses for