Analog Computers

In her excellent book, Longitude.  Dava Sobel told how a clock behaving in a deterministic way is a crucial component of successful navigation. The first accurate clock which could maintain synchronisation with the time at zero degrees longitude was the beginnings of an analogue navigation computer, albeit one that needed a little manual assistance to complete its task.

What makes an analogue? An old style vinyl album records a representation of the sound waves that took place when the original piece was recorded. A frozen graph of the sound that’s been curved into a spiral from the outside to the centre of the record. The volume level of the sound is represented by the depth of the peaks and troughs of the graph, and the pitch by the interval between the peeks and troughs. Low volume sounds tend to be hard to distinguish from imperfections in the manufacture of the disk - noise. The primary characteristic of analogue systems, and their main limitations: the magnitude of all values is represented by the magnitude of a signal somewhere.

The schematic above shows the Moniac analogue computer. (currently on display in the Science Museum in London.) It was used as a teaching aid to model the national economy. Moniac used the flow of water through the system to 'model' an economy. It featured valves to create the effect of changes in taxation and other policy decisions. See Monica technical description for a complete article.


Analogue computers used in gunnery and navigation need to be built like the proverbial brick outhouse. They generally used gears and motors. Again physical values are used with, for example, the distance turned by a wheel representing values such as speed or distance.

A number of mathematical functions must be performed and over the years engineers have dreamt up different ways of performing them. A classic problem in air navigation is distance travelled over ground when all we know is airspeed and direction. Before we get very far we need to do a little Polar to Cartesian conversion. If airspeed is known, and heading, how can we get distance travelled north and south, east and west?

One answer was the ball resolver, see below. Essentially it is an infinitely variable gear. (This is actually from an early 20th century computer in the Science Museum for predicting tides but the same mechanism in somewhat miniaturised form featured in many navigation computers.)

An electric motor turns the disc at a speed that represents airspeed. The disc turns the brass ball and the ball turns the roller. If the ball is positioned at the outside of the disc the roller will turn quickly, if the ball is moved towards the middle of the disc it will turn the roller progressively slower.

The system has another motor, this driven when the aircraft heading changes. This motor moves the ball from side to side. When the plane is flying due north the ball is positioned at the outside of the disc. The roller is turning at it’s highest possible speed for the speed of the disc. If the aircraft turns to the right and flies more east, as in the diagram below, the system will move the roller towards the centre of the disc. Even with the airspeed the same the velocity north will reduce.

A second resolver is used to calculate velocity east. Identical to the north one except that the ball is set up to move differently. With this one the ball is at the outside of the disc when we are flying east and at the inside when we are flying north.

With the two revolvers we can convert the polar parameter airspeed and heading to the cartesian velocity north and velocity east. And velocity north goes negative when the aircraft is flying south and velocity east goes negative it is going west.  (and, BTW those same resolvers can work the trick backwards and convert Cartesian back to Polar.)

With the velocities it’s pretty easy to calculate distance travelled. Those mechanical counters on bikes and speedometers integrate speed and get distance. It’s just a matter of gearing. If we’ve decided that 60 miles an hour is represented by 60 revolutions per minute we just have to arrange the gearing so that one mile will pop up after 60 revolutions of the velocity rollers.

We can even turn distance in miles into degrees of Latitude and Longitude. 60 nautical mile is one degree of Latitude, but Longitude is a little more tricky. Those pesky lines of Longitude get closer together as we move away from the equator so a third disc resolver needs to be added to correct the velocity East/Longitude, for changes in Latitude.

Of course, these calculations assume no wind conditions, unlikely. But supposing we have a system that can measure groundspeed and drift. Moving on a few years from the Norden bombsight etc (which needed ground observation to get drift) we had aircraft like the Vulcan with downwards looking doppler radar that could directly measure groundspeed and drift. So, instead of feeding the computer airspeed we feed it groundspeed derived from radar, Green Satin.

Green Satin  also supplies the drift angle, the effect of wind on the aircrafts track across the ground. In order to complete our analogue computer we have to find a means of adding two angles together, the original heading angle and the drift angle. For such an apparently simple problem the mechanical analogue computer  uses a differential. Two input values, one for heading angle, one for drift angle are (algabraically) added to produce track. Track is the angle of the aircrafts path over the ground.

All the above techniques are only a sample of what was done with electromechanical analogue computers. The mechanical systems have their shortcomings and there are numerous other analogue techniques, many of them purely electronic. Vacuum tubes amplifiers were used, and as soon as they became available, transistors. And I’ve not even mentioned a now totally obsolete lost world ‘solid state’ technology called magnetic amplifiers.

Now the analogue computer has had its day, although it arguably reached it technical best, (with integrated circuit operational amplifiers) just as it was being made obsolete by the digital computer. As Robert Heinlein observed, that tends to be the case, by the time a technique is finally perfected, it’s generally obsolete. And so it is with the analogue computer.

Bomb Sights

The first bombing raids on London, by the Graf Zeppelin in 1917, democratised warfare as never before. The killing was know no longer confined to the battlefield and  soldiers but now included cities and the civilian population. By the nineteen thirties the prospect of city bombing had gathered pace to the extent that Britain's RAF invested greatly in bombers. (Far more so, in fact, than in the fighters  and radar that would eventually win the Battle of Britain.) These new bombers were to be a deterrent force that would make attack unthinkable.  But when the war began it was soon realised that the accuracy with which the bombers could deliver their payload was quite another matter.

At the start of the Second World War, aircraft navigation, especially at night, could be extremely poor. Prague was bombed only once and that was by accident. The pilot who bombed Prague thought he was bombing Dresden, an error of over a hundred miles.

Then, assuming that the target was found, there was the issue of bomb aiming. In the early days this had been left to the pilots judgement. As aircraft altitudes and speeds increased bombing effectively became more difficult. The fundamental problem of air navigation is the difference between the aircraft's motion through the air and its motion over the ground. To bomb accurately the motion over the ground must be known, but the aircraft instruments of the 1930s and 1940s could only determine motion through the air.

The instruments of the time could measure airspeed with reasonable accuracy and heading can be found from a magnetic compass. But, because of wind, speed and direction over ground rarely corresponds with speed and direction through the air. The air is usually in motion, and any calculations regarding distance travelled, based only on speed through the air and direction pointed will be subject in error. In order to correct speed over the ground, and track (the path of the aircraft over the ground) wind speed and direction must be known.

The first generation bombsights attempted to add in a predicted wind speed based on weather forecasts. But the forecast winds were often way out and the vector sights which allowed the bomb aimer to ‘dial in’ the predicted winds were soon found to be inadequate. But aircraft motion over the ground could be determined by observing and tracking a point on the ground.

The Norden bombsight was one of several computing bombsights developed by the USA, Britain and Germany which were capable of deriving the wind speed and direction from observation of the target. The Norden bombsight was probably the most hyped weapon of the Second World War. It was an improvement on the vector sights its true accuracy was nothing like the claims made of it. And, long after its secrets were well know in Germany, crews had to carry out a post mission procedure of locking the bombsight up that was as useful as the ceremonial changing of the guard outside Buckingham Palace. Germany had long had a comparable bomb sight of its own, the Lotfe 7. And this was well known to the Allies from Lotft 7s found in shot down aircraft.

The RAF precision bombing experts, 617 squadron, used a British designed sight, the SABS (top picture), and very effectively. At their best, 617 could deliver precision weapons such as Tallboy to an accuracy of 125 yards. The Tallboy bomb, devised by Barnes Wallis, was used to bomb the Tirpitz and against the Saumur tunnel.

All these bombsights were tachymetric sights -calculating speed (windspeed) through observation.

Initially the bomb aimer set the known vertical separation (altitude over ground) between the bomber and the target into the bomb sight computer. He then had to manually align a telescope, which was part of the bombsight, on to the target. The bombsight's computer, which was getting airspeed and heading, then continuously calculated a pointing angle for the telescope as the target was approached. This pointing angle steered the telescope through a servo mechanism.

In reality this would only work in ideal, no-wind conditions, and in practice, the target drifts across the field of view of the telescope. The bomb aimer has two controls which he must use to 'null out' the drift. These controls set up a 'vector' in opposition to the actual wind vector, (which is the speed AND direction of the wind). With the drift correctly neutralised the telescope will automatically track the target. And the adjustment of the drift controls has produced values for the direction and strength of the wind. With the wind known the bomb release point can be determined with much greater accuracy.

The American and German tachymetric bombsights were designed for use with aircraft with autopilots. They produced a steering signal which was fed into the autopilot to correct the aircraft heading. An automatic bomb release signal was also produced. The British SABS bombsight drove director lights which the pilot had to follow.

In an earlier blog I mentioned how the guidance technology developed in the Second World War was developed for use in missiles and, eventually, the Apollo moon landings. The technique of tracking an object, manually on a bombsight, was, within ten years fully automated. ICBMs used  automatic star tracking to achieve great accuracy. The submarine launched missile system Polaris is so named because of its ability to automatically track the Pole star.

By the time engineers and airmen had achieved great accuracy the development of nuclear weapons seem to have made accuracy obsolete. The reality has turned out a little differently, bomb accuracy continues to be improved with guidance such as GPS and laser now achieving accuracies of better than 13 metres.

Waiting for the Telegram

In print the short story can be spare and effective, but in the face of stiff competition from other media it is no longer hugely popular. In his collection of TV monologues, Talking Heads, Alan Bennett has devised a TV format which is very similar to the short story form in its impact, and with the same demands on the writer for economy. Each story is told by a single actor, talking straight to the camera and telling the story. When talking to strangers intimacies can be revealed which cannot be shared with family or friends.

In Waiting for the Telegram Violet tells her story. She is played by veteran actor Thora Hird. Violet is a lady in her nineties, if she lives long enough she’ll receive a telegram from the Queen congratulating her on her hundredth birthday. The story looks back over her long life and the priority that her failing mind has given to various events. This is a brilliantly constructed piece which unfolds most elegantly, which you can see here. Waiting_for_the_Telegram

Violet is hospitalised with some other elderly people and often has trouble finding words. She's been coached to describe a thing when she can't remember the word. Using this method women's breasts become,
                                 "Them two things with pink ends that men like."

Alan Bennett uses this device to put the telegram into an additional context. When Violet can't find the word for telegram she has the line -

            "What is it now? Lad comes on a bike. Folks stood at the door weeping."

Commercial telegram operation began in 1839 and telegrams were used to mark great events in family life. Births, marriages and deaths were announced by telegram. In the First World War the first news that a loved one had been killed came via a telegram. Violet’s generation was promised a war to end wars in 1914 and the memory of the loss of so many young men is still vivid to her.

When she's asked, "What war?"

  

She snaps back angrily, "The proper war. When all the young lads got killed. Never again. That war."

Despite the loss of more recent memories she can still remember the last time she saw her sweetheart. He and her had been left alone together on his last night before he went off to war. Somehow Violet’s sense of propriety, or her shyness, inhibited her. Her sweetheart went off to war unsatisfied. She has regretted this ever since.

And this is a masterly performance from Thora Hird. Almost ninety at the time, and playing a character ten years older. As the tale unfolds she deftly switches voices to recite the words of the nurses and the other inmates. And, despite the sadness of the tale, Bennett still gives Hird a few funny, surrealistic yet typically northern lines. She delivers these with superb timing. - Violet is recalling how Rennie, another elderly, deluded lady inmate who is always telling people that she’s waiting for a taxi.

"Rennie, where is this taxi going to take you?" Violet asks her.

"To my mam and dad's in 1947." says Rennie.

"Well if he can take you there I bet he does a spanking trade."

We hear of the indignities that the elderly patients are subject to: being dressed in the wrong clothes, even given the wrong teeth. But we see the affection that Violet feels for some of the staff. And in particular one of the male nurses, Francis. And we discover that a lady of ninety can still look at a young man's body and admire it.

Then, at the end, after Rennie has died, "well he (the taxi) came in the end." Her nurse friend Francis dies too. By now Violet has had enough, we sense that she won't last much longer.

A telegram brought the United States into the First World War and finally brought the massive slaughter to a close. The Zimmerman telegram was an encrypted communication between the German Foreign Secretary and the German ambassador in Washington. It was intercepted by the British and decrypted by Room 40, Britain’s cipher decryption centre. The conflict finally ended in 1918. Yet it was only twenty two years later that the lie of a war to end wars was revealed and the Second World War started.

Bennett finds a new way with the short story form that was hugely popular in 1918. He gives it a facelift and preserves the superb performance of a fine actor. Waiting for the Telegram is something very special.