Category Archives: Ramblings

Thoughts on interesting, amusing or annoying topics

Time to ditch the landline?

My last post mentioned that BT are planning to switch off the landline telephone network in five years (in 2025).  They haven’t yet told us how this might affect users, and indeed, I can’t find anything about this subject on the BT website.  However, Ofcom issued a document in February 2019 which sets out some of their requirements.  This says that ‘major landline providers’ (i.e. BT in most of the UK) ‘will offer VoIP products delivered over broadband … before PSTN is retired completely.’

Ofcom say that whilst 81 % of households have a landline service, ‘most’ consumers now have a mobile and use it as their main method for making and receiving calls at home.  4 % of adults have a landline but no mobile but only 3 % have a landline and no broadband.  There is possibly more concern about non-voice landlines, such as those used for alarms and payment systems, that rely on attributes of the PSTN that may not be replicated by VoIP.

BT have to an extent already presaged the switchover by no longer charging separately for ‘line rental’ on their broadband deals.  However, they are not yet offering any VoIP ‘landline’ option. Their traditional landline offering is quite expensive – ‘unlimited’ anytime calls cost £9.99 a month (less if you have a broadband deal) but if you go over the one hour (even by one second) on any call, they whack it up to the standard rate of 15 p per minute.

They did have a system called ‘SmartTalk’ that let you talk over your broadband wi-fi from any mobile phone when at home or within range of another wi-fi system, but this was very quietly dropped in January 2019.

Their overt reasoning was that it was no longer needed because mobile plans had generous airtime and less onerous roaming charges. A more likely reason is that they started to offer ‘wifi calling’ on BT mobile phone contracts, so they’re making this an advantage of their system.

Along with their offering of extra data allowances on BT Mobile if you have broadband with them, they are pushing BT Mobile very hard.

In the early days of mobile, I got one from BT, but I switched to O2 when they stopped offering up-to-date handsets. Recently, I made the foolish mistake of getting a new BT Mobile phone, but it couldn’t get a signal inside my house.  When I complained, their advice was that I should go outside to make a call! Needless to say, I had to send the phone back at my own cost, including insurance!

My present Samsung phone works fine on O2 indoors, although admittedly with a weak signal, and its calling plans when abroad (I travel for business and pleasure) are cheaper than BT.

Sadly, I think it is time for me to get another SIP adaptor for my broadband and to port my landline number across to it.  Sipgate charge 1.18 p per minute with no contract, or you can get various deals which are reasonably priced.

Previously, I’d thought that BT were competing by offering advanced and reliable technology, albeit at a price. In the early days they offered ‘Broadband Talk’ and they even provided me with a broadband phone.  But that has also gone without explanation.

Now I’m finding them to be putting profit first and I’m starting to doubt them.  Shame, as I’ve been a very loyal customer, ever since as a student, I trained in various telephone exchanges in Manchester.


End of the old telephone system

Strange that after yesterday’s gripe about BT, I read that the old ‘copper pairs to the exchange’ telephone system (called PSTN) is to cease in 2025.  You may not be able to get a new ‘telephone line’ after the end of this year!

I’ve read very little about how this will affect people who only have an old ‘sit up and beg’ hard-wired phone.

People currently using the BT network for broadband and voice actually have two services provided over the one pair of wires coming into their premises.  One is the original PSTN that connects to their local exchange, and the other is the high-frequency broadband signal that in many cases is fed into your own pair of wires in a street cabinet close(ish) to your premises.  It probably isn’t fed in at the exchange, as this may be too far away for a high-frequency signal to reach your premises.

So it would be simple to turn the power off at the exchange, which would mean the PSTN will stop working, but the broadband will continue.  However, the traditional ‘sit up and beg’ phone relies on the power supplied from the exchange, and these would no longer work.

I have used a VoIP (Voice over IP) system for over a decade for my, and my partner’s, business.  There is a cheap and simple Linksys VoIP adapter that plugs into the broadband router and has two ordinary phone outlets giving me two phone numbers.  I subscribe to a VoIP provider (Sipgate) which costs very little (certainly compared with BT’s charges) and the system works very well.

You can, of course, buy a phone that has the VoIP adapter built in. In this case, you simply plug the phone into a broadband socket, and it works in every respect like a dial-up phone. No doubt this is what BT will provide to people in future, if they just want a phone.

Of course, you still need to have a phone number that people can dial.  This is set up by your VoIP provider, Sipgate in my case, but no doubt BT will provide a number in future.  Indeed, I did have an internet phone provided by BT with one of their original ‘Home Hubs’, but clearly they didn’t want me to use it, as charges were higher than for the landline, so guess what? I eventually put it into the recycling!

The monthly instalment payment rip-off

I’ve used BT as my broadband internet provider for at least 20 years, and I’ve had a BT landline for much longer than that.  I used to pay off my bill in full as soon as it arrived, which was quarterly in the days before on-line billing, and later to monthly billing. My ‘deal’ costs about £60 per month.  Quite expensive, really, especially when compared with my mobile phone bill, but the speed and reliability have improved hugely over time, and changing could cause problems that would offset the savings I might make.

Well, I try out various new things for BT, and in May 2018 they asked if I would try Quarterly Billing with a monthly payment plan and I naïvely agreed. They set up the bill to be issued on the first day of each quarter and for the direct debit to be taken on the first day of each month.  They told me the payment would be £49.50 a month (which didn’t seem right to cover a bill of about £60 a month, but I left it to see what happened), starting from 1st July 2018.

So, with the first bill issued on 2 Jun, I was £177.61 in debit because they charged for 3 months of usage, and no payments had been taken.

By the next bill,  1st September, they told me I was £181 in debt.  How come? I checked and they were taking £89 per month – not the £49.50 they’d originally said – but there were only two direct debits – 2nd July and 1st August.  Of course, the 1st September payment hadn’t been taken because that was a Saturday, but they were charging me for 3 months in advance.  So by the next bill, 1st December, my account was £103.93 in debit.  There were 3 payments of £89, but the payment of 1st December had not been recorded – again a Saturday – so they increased my payments to £99.50 a month (remember that my usage was about £60 a month). By the next bill (March), I was £99.42 in CREDIT, but they kept the payments at £99.50 a month!

The catch is that with quarterly billing, they charge you three months IN ADVANCE and at the same time, they expect you to have built up that 3 months credit BEFORE they calculate your account.  If you don’t have enough to clear the account at that point, they whack up the monthly payments.

But that’s not all: even though my account was already almost £100 in credit, they only reduced the monthly payments to £78 a month even though I’m spending about £60 a month!

So they have built up a credit of over £300 in my account, so that they can pay off a bill of £185 which will itself be charging me for three months in advance!

It should be noted that many other companies are doing the same. Steer clear! Pay your bill in full every month. It will cost you less.

Time Zones

Anyone following this blog will know that the time displayed on a clock depends on its time zone.  Time zones are intended to allow everyone in a region to have a common time reference.  The convenience of this overrides the slight inconvenience arising from midday (and thus sunrise and set) getting slightly later as you move to the west.  And of course, there is also a north-south variation in sunrise and set.  The length of the day gets shorter as you move north in the winter, but longer as you move north in the summer, due to the tilt of the earth.  This means that in the winter in Scotland, it doesn’t get light until about 9 am.

The EU imposed a common set of time zones accompanied by ‘Daylight Saving Time’ in 1996.  However, the moving of clocks forward and back in Spring and Autumn is unpopular and the EU is now proposing that Summer Time should apply all year round.  The change would mean lighter evenings but darker mornings. Apparently, this was most popular with Germans and Austrians, but Britons and Italians didn’t care.

Permanent summer time was tried in the UK, with one argument being that it would reduce road accidents and energy consumption, but there wasn’t much evidence for this especially as some parts of  Scotland and northern England wouldn’t get daylight until 10 am in the winter.

Speaking personally, I quite liked the permanent summer time because of the lighter evenings.  Being in the south of England, the dark mornings are less of a problem than the dark evenings, although I have to say that in the winter months, there just isn’t enough daylight and messing with the clocks won’t change that.

And speaking as someone who looks after a church clock, I would rather not have to turn the clock hands on an hour in spring and stop it for an hour in the autumn. (It is possible to advance a chiming clock, but not to wind it backwards, as this would damage the mechanism.) The clock is mechanism is shown at the top of this blog.  It is a typical English flat-bed design, made by JW Bennett of Ludgate Hill in 1871, so it is almost 150 years old.  Nevertheless it is very sturdily built and keeps accurate time with little adjustment.  The frame is of heavy cast iron about four feet wide and 2 feet deep.   I still wind it by hand, having resisted the blandishments of those who would ruin it by fitting electric winding motors.  The weights are extremely heavy (I calculate that the quarter chiming weight is almost one ton and must be wound about 40 feet every week) but two fit people can wind it in fifteen minutes.  Adjusting the clock is a bit of a fag as the time shown on the mechanism does not exactly correspond with what is shown by the external hands, so unless I’m very careful, I have to climb the tower several times to get it right! It has a long, heavy pendulum at the other side from the photo. This goes through the floor into the bell-ringing chamber underneath.


Is this another idea that had the best of intentions but which has backfired badly?

Almost every time I log into a new website I am alerted to their ‘Privacy Policy’ and told I must agree to it before proceeding.  One effect is to make me head for another website, but sometimes they seem to have just the information that I need, and I decide to bite the bullet.

Have you ever read these ‘Privacy Policies’?  After the initial dishonesty of telling me that my privacy is important to them, they ask me to agree to waive it.  Usually, I take a look at the policy, and it is clear that the real desire is to milk me for everything I’ve got.

They want to track my precise location.  They want to use my search history.  They want to know what other sites I’ve visited.  They want to send me ‘targeted advertising’; they want to influence my search results. They want to let their ‘selected partners’ know what I’m up to.  How do I know what else I’ve unwittingly agreed to them doing?  Have I allowed them to put on a keylogger, to turn on my webcam or microphone?  For all I know, some of their legal gobbledegook might permit this, or even nastier things.

It’s likely that all this was going on before GDPR, but at least without my permission. There was always the thought that they might be wary of going too far.  Now they have my explicit permission.

Have you tried going through their privacy policy and tried turning off all these permissions? If you did, you almost certainly ran out of time and patience, because these are so confusing and slow that you would never get to the end whilst retaining your sanity.  Sometimes they are worded so that ‘no’ means ‘yes’, for example.

I suppose it must be worth their while, but for the most part their ‘targeted advertising’ is about as accurate as a blind bowman with a bent arrow.  I get adverts for rooms in hotels in places that I never visited and never want to visit.  I get flights to places that I went to months ago.  I get products that I have no interest in acquiring.  I get offers for so many printer cartridges that I could open a shop.

Some adverts are so annoying and distracting that I can’t use the website they’re fixed to.  I do occasionally delete tracking cookies, and should probably get this to run automatically each night.  I would rather pay a small amount to look at genuinely useful websites, just like I pay for newspapers and magazines.

On the other hand, it does mean that I’m strongly biased to ‘professional’ websites which generally have more reliable information, rather than clickbait.  OK, I may sometimes have to pay, but my time is precious and if it means that I can avoid some of the dross, all well and good.

I conclude that GDPR has resulted in us signing up to things we would have previously avoided like the plague.  Result!!!

Smart Meters – Updated

An update on Smart Meters – January 2020

I’m having my house renovated and this requires my incoming electricity cable to be relocated to a new external meter box.  The network company will be doing this tomorrow.  However, they say they are not allowed to reconnect the smart meter – only the supply company that I pay for electricity can do this. I transferred my account to a new, small supply company some time ago, and when I eventually got through to them, the guy I spoke to didn’t know what to do. I phoned again and clearly he’d put the problem on the ‘too difficult’ pile.  After several further calls (all involving long waits on the line) and emails  I got through to a helpful girl who said that they needed at least two weeks’ notice.  Luckily I had copies of my emails showing they’d had much more than that but hadn’t dealt with it.  Now, it seems that they are going to have to call out the meter fitting company as an ’emergency’ as the house will be without heat, light or power once the cable is disconnected.

I’m sure the story can get worse, but fingers crossed that I’m wrong. I wonder if they will fit a SMETS2 meter? I bet they don’t!

Update November 2018:
A recent (Nov 2018) article in ‘Private Eye’ describes the real purpose of ‘smart meters’: they will allow retail electricity pricing to vary every half-hour.  The idea is to regulate demand, thereby reducing the need for booster generators and to maximise the use of renewable energy at times when it is most plentiful.  For example, you can charge up your electric car or boost your hot water, and are thus storing energy locally, saving the power companies from investing in storage and reducing the peak load on the power grid.

However, there is opportunity for ‘Big Brother’ to probe further into our lives: apparently researchers at Liverpool John Moore University have been looking at usage records and noted that if someone is turning devices on and off randomly, this could be evidence of dementia.  Of course, the honest and true person has nothing to fear!

The benefits of Smart Meters?

Back in 2016, Scottish Power bombarded me with material implying that smart meters would be compulsory by 2018 and I should take the opportunity to get one fitted whilst they were ‘in my area’.

There was the usual blather about how quick (no more than an hour or two)  and easy it would be to get it fitted and how much money I could save.  So, foolishly, I agreed. The fitter came at the time appointed.  I’d already emptied the tins, bottles and gadgets from the kitchen cupboard and powered down my computer and network – not particularly quick or easy –  so he could get to work immediately.  After a while, he didn’t seem to have done much. I think he’d taken out the company’s fuse. The new meter is in a small plastic box which easily fitted into the space on the fuseboard, where there had originally been a large old-fashioned meter. I’d had the consumer unit and wiring replaced not long before, when I’d had my kitchen re-fitted.  After quite a while, he fitted the new meter onto the existing tails from the company’s fuse.  But something was bothering him.  He said he would have to fit new tails,  I have no idea why, but I rather felt that he thought it might be possible to somehow bypass the meter.  It took an unconscionable time for him to do this, but after elaborately fitting tamper-proof tags onto all the covers, he pronounced himself satisfied.  It was time to replace the gas meter, which is in a box outside.  This meant taking the box off the wall, which was a big job as a vine had gone behind the box and entwined itself through various holes.  After I’d cut the vine off (not his job), the box came off the wall and the meter was replaced. Was that job done?  You’re joking!

After doing further elaborate leak tests, which I had to witness and sign for,  it was now time to get the gas meter to talk to the electricity meter and for the electricity meter to talk to the remote station, via the GSM network.  After fiddling for ages with an app on his mobile, he called ‘Sandra’ at the electricity company, and after a considerable amount of discussion, several long waits and more calls back and forth, it was set up.  The ‘One or two hours’ had become four or more hours and he was well late.  He needed to get home, so he shoved a badly photocopied sheet of illegible instructions into my mitts and scarpered.  I don’t blame him.

Now, I had been assured that this smart meter would save me money.  Hundreds of pounds.  I don’t know how it was going to achieve this.  Instead, it almost cost me a divorce!  I said to my partner, ‘Oh, look, that red light shows we are using a lot of electricity’.  To which her reply was an uncompromising, ‘Well do you want your dinner, or not?’

In fact, it did show that electricity is only a small part of our energy consumption – it is gas for heating that is the big expense. Well, I already knew that, and had already turned the thermostat down as far as I dare! To some extent, doing that was self-defeating, as we seem to have mysteriously acquired several electric fan heaters and a new duvet!

But at least I wouldn’t have to be bothered by meter-readers, as the energy company would get the readings remotely.  Oh yeah?

Within a few days, a meter reader turned up.  ‘No, Sir, we have to read your meter regularly, it’s the law!’

Not long after, Scottish Power put their prices up by a serious amount and I decided to shop around.  I found a small new supplier who was about the same price as Scottish Power before the price increase.  ‘You will have to submit monthly readings, Sir’.  ‘No, I have a smart meter, just take the readings from that.’  ‘Sorry, Sir, we don’t do smart meters’.  ‘That’s OK, it has already been fitted, you only need to take the readings.’ ‘Sorry, Sir, we don’t have the capability.’

I phoned several other big suppliers, and learned to my disbelief that the smart meters are dedicated to individual suppliers!!!

This is incredible! Since you can save far more money by switching supplier than by taking a cold shower and sitting wrapped in damp towels with a cold supper in a freezing house, why has the government permitted this massive disincentive to switching?  What a con.  And the cost has been foisted on us, the consumer! I’m aware that there is supposed to be an upgrade ‘in the future’, but this has already been delayed twice and given the trouble that the fitter had, will it even work?

For a while after switching, I was still able to get the meter readings from the ‘in-house display’ but now the gas meter reading is not changing.  So I have to go outside, open the meter box and try to find the meter reading. On the old meter, there was a big, bold, cyclometer display, unmistakeable even in the dark.  The new meter has a tiny LCD display that seems to show a lot of gibberish.  It has left and right cursor buttons but it is entirely unclear what they do or what the display shows.  After a few random presses, I found a number that looked like a plausible meter reading (see the image above) and sent that off to the company.  No doubt the burly meter reader will be round before long. We’ll see what he says.

How can the government get away with this?  We’re mugs!



Elan Valley Dams

By comparison with my rant on the Ironbridge, I must mention the Elan Valley dams.  Again, there is fear of anything technical – the Visitor Centre majors on the wildlife and walking, which are commonplace, whilst the dams are unique.  But we were lucky enough to be there on a Dam Open Day.

This did not mean that they opened the floodgates!  We were able to walk along the central tunnel within the dam to the central control tower.

There we could talk to an interesting young female professional engineer about how the dams are operated and controlled.  She had an enormously responsible (and quite scary) job walking around inside these dark, dank and noisy places (rushing water is very noisy) operating various valves within these vast dams to control the water levels and the supply to the cities of the Midlands and South Wales.

This is real Victorian engineering working much as the Victorians left it – the valves are still operated manually by turning hand wheels.  It’s a truly fascinating world that would interest any youngster. For example, the pipe to Birmingham is quoted as being 60 inches in diameter and 70 miles long. Converting to metric units, the pipe has a cross-section of 0.75×0.75xπ = 1.767 m² and a length of 70×1.6=112 km. This is a volume of 1.767 x 112000 = 197,900 m³.  And since a cubic metre of water weighs one tonne, the weight of water in the pipe is 197,900 tonnes!  Even though it is only flowing at 1 mile per hour, you can’t stop that easily or in a hurry.  I would have thought that problems such as that would inspire any intelligent child.

I like to think that my daughters, who have highly responsible, difficult, technical jobs, were inspired by the things I was able to show them as kids.  And by that I don’t mean dressing up in Victorian costume, although they enjoyed that as well.

The whole flow runs entirely under gravity, even though there is only a small height difference of 52 m between the beginning and end of the pipeline, an average gradient of 1 in 2200.   Of course, some parts of the pipe flow uphill and the tops of these ‘siphons’ are fitted with air valves.  By opening these valves, the uphill pressure and the downhill suction are relieved, thereby allowing the water flow to be broken.  Even so, I gather that the pipes were plagued by fractures, especially in the early days.  Cast iron is quite weak when under tension or shearing forces, and sections have the pipeline have been encased in concrete to strengthen it.

The illustration shows the Craig Goch Dam (Red Rock Dam), which has a roadway across the top rather than a tunnel through the middle, but is equally exciting to visit.

World Heritage Sites

Is becoming a World Heritage Site the worst thing that can happen to an area?

I’ve been visiting a few places recently, not because they are World Heritage Sites, but because they are of interest to me. One such is the Coalbrookdale site near Ironbridge.

Ironbridge is undoubtedly of world importance, as it was the place where  high-quality cast iron was first produced in industrial quantity, due to the development of a blast furnace fired by coke rather than charcoal from about 1709.

When I first visited a decade ago, the blast furnace (made of brick) had been enclosed to protect it from further weathering, but much of the site was in a state of genteel neglect, and it was fascinating walking round and doing a bit of amateur industrial archaeology.  In fact, although the blast furnace had closed maybe 200 years earlier, an iron foundry was still operating on the site.

Now, the whole area seems to have been taken over by its ‘heritage’ status.  I had hoped that this might have improved the area, but sadly the whole area seems to have fallen into the thrall of heritage status and the costly need to entertain the masses thus attracted.

Take the ‘Museum of Iron’ as an example.  The home page of its website confronts you with a casting of a dog, a fountain and a woman. Now, these are undoubtedly fine pieces of art, but I can go to dozens of museums, stately homes and public squares to look at similar.  At the museum of iron, I would like to see descriptions of the chemistry involved, the way that castings are made, and the uses to which cast iron has been put, and even when it did not work out, such as the Tay Bridge disaster.

The fame of the area is said to result from Abraham Darby developing a patented foundry method that enabled him to produce cooking pots better and more cheaply than his competitors, and to produce other fine castings.

Do you get anything of an engineering or technical nature that explains the problems that the great Abraham Darby had to resolve in building his blast furnace?  Oh, no! Nothing that would frighten the horses.  According to the website and brochure, rather you can see the ‘Eagle Slayer’ statue, have a picnic on the green, or a selfie by the Deerhound Table! Or you can hear the clock bell chime!

This is pathetic.  You get the feeling that it has been put together by a ‘professional heritage manager’ – keeping those boring engineers away from the public. Oh yes! Engineering is rather unpleasant, dirty stuff from which the public has to be shielded.  There is even an engineering art museum, which has a lot of beautiful, arty design drawings, but nothing of how the objects are made.

I asked at the desk and they said there was something more in the ‘Enginuity’ exhibition, so I toddled over there.  I found that  it is a children’s ‘discovery play’ area, with educational interactive things to do, but !!Problem!! They (all) close at 4pm – yes even in the peak school holiday period – and given that it was as late as 3.30, I was told it was too late to go in anyway.

I was advised that I’d be better off going to the Blists Hill ‘Victorian Town’ the next day, which I did.  Now I was brought up in Rochdale.  I walked to school past mills with open doors emitting (literally) deafening  whirring noises from spinning mills and clackety-clacking from weaving mills. The smell of oil and cotton dust permeated the streets.  I had to dodge the horse droppings when crossing the road – which was particularly difficult in gas-lit streets after the smog had come down.  And – perhaps this is not something that it is wise for me to mention – the village (i.e. shared by several houses) toilets were wooden pails that were emptied by men who came in a lorry.

This ‘Victorian Town’ consisted of a few people wearing old-fashioned clothes in shops with old-fashioned frontages and with some old-fashioned products on their shelves.  I didn’t see any women donkey-stoning their front door steps, I didn’t see any washing on lines, or women doing their pegging-out.  I didn’t see a wash-boiler even.  And the biggest give-away – most of the ‘actors’ were wearing trainers! I didn’t see a single pair of hob-nailed boots or clogs.  What about the coal-men with smudged faces carrying their shiny black sacks of coal? It took an awful lot of coal to heat those houses and run those steam engines.

In fact there were sheds with line shafts along the  roof space and lots of old machines, but almost none of them had a label to say what they were.  It would have needed no more than a few days to make some labels.   My suspicion is that the ‘curators’ simply do not know or care.  After all, it costs a lot to run an enterprise like this, so it ends up as cheap entertainment for the masses.  And far from educating, I think it misleads the public by doing that.

AC v DC mains

UCL & power station location
UCL & power station location

In my last post, I got onto the subject of Rectifiers, which caused me to remember the late 19th-century debate about AC v DC mains supplies.  When I was a student, the original Engineering Building of my college – University College London – was in Gower Street, immediately opposite the famous University College Hospital.

The Engineering Building was a place of wonder for me: it was filled with seemingly-ancient engineering marvels – a steam engine, an oil (diesel) engine and all sorts of materials testing machines for pulling, squashing, bending and twisting bars of steel and other materials to see what happened.  There were also machines for ‘fluid mechanics’ to test Bernoulli’s equation, viscosity, Venturi and the like.  These were serious industrial machines and we got to play with them – although of course, being engineers, the serious purpose was to understand their operation in mathematical terms. And we did.

Being housed in the basement of a grand Portland Stone building, dating, I presume, from the early 20th century and within a short walk from the British Museum, the accommodation was cramped, and for convenience even the steam engine used an electric pump to create a vacuum on the condenser (NB – the steam condenser, not an electric one! – Oh, sorry, they’re not called condensers any more.)

But – the point of all this – all the electric machines ran off the DC mains.  I repeat this – the building was supplied with 220 v DC mains from the street!! Yes, in 1964, the street still had a DC mains supply. [See the end of this post for more information.] Of course, it had AC mains for most of the building, but DC was available for the electric motors.  This is not the place to discuss the advantages of DC for electric motors, but of course it is still used for traction motors (underground trains, suburban lines and trams in particular).

It is simpler to control a DC traction motor than an AC motor, and this was one of the arguments in favour of DC.  Another important argument was that DC can transmit more power than AC for a given voltage: in a 240 v mains supply, the peak voltage is √2 times the RMS voltage (the ‘average’ voltage, if you like) i.e. it is 1.4 x 240 = 340 v.  But, 100 times every second, the voltage is zero!  Especially before the invention of plastic, it was really difficult to find a durable insulator for the cables: indeed this was one of the greatest bugbears in the development of the electric telegraph.

Originally, the generating stations had DC dynamos, but as AC generation became the norm, DC was produced by using a motor-generator set – an AC motor driving a DC generator. But these need housing in a special building and require constant maintenance.  For a small application – a lift motor for example – the answer was a mercury-arc rectifier.  I saw a number of these in action, although I can’t quite remember where.  I suspect it was when I worked for the Post Office Telephones. They were fascinatingly eerie things to look at – a fiendish blue glow and a bright arc dancing around inside a large glass octopus.  Probably emitting dangerous amounts of UV and X-rays, perhaps.   Yikes!

TBH I don’t know how they do it now: large radio transmitters need kiloWatts of DC power usually at high voltage.  In former times, this would be obtained from a vacuum-tube diode, sometimes using a mercury-vapour device. Clearly, I need to do more research.

BTW all the electrical engineer’s machines at UCL were housed in the ‘new Engineering Building’.  Here, we had what were essentially motor-generator sets, but all the windings were terminated in terminal blocks and they had to be correctly connected to emulate a variety of different set-ups.  It’s interesting that so much emphasis was placed on ‘heavy-current’ engineering: although this is still essential to the modern world, everything has been overtaken by digital electronics – a subject that was still a matter of esoteric research when I was a student. A time when transistors were weak and unreliable and an integrated circuit was the size of a packet of cigarettes (if anyone can remember those!)

Thinking of all the safety warnings we get nowadays, I wonder if students would still be allowed to do the sort of experiments we had to do?  The motors must have been powered at the equivalent of mains voltage and the wiring came out on bare terminals that we had to cross-connect.  I recall that when setting up generators, we had to use a ‘loading frame’ – essentially a three-bar electric fire – in order to test the on-load characteristics of the generator. This must have been running with a significant voltage and current, but I don’t recall any accidents.

Yes, we did study electronics, but this was almost entirely theoretical – the mathematics of high-frequency reactive circuits involving ‘complex’ maths – i.e. with ‘real’ and ‘imaginary’ parts – being very mentally challenging! I was never completely happy with grad, div and curl functions. Good old Bessel 😉

More information on Historic Power Stations in London
The existence of a public DC supply to University College London, Gower Street, London WC1, as late 1964, merited further investigation.

UCL is in the West End of London, around 800 m south-east of St Pancras Station and about the same distance north of Oxford Street and theatreland – Leicester Square, Shaftsbury Avenue, Covent Garden, etc. It is in the area that was known as Bloomsbury when I was there, but is now shown as ‘Fitzrovia’ on Google Maps, presumably due to the migration of trendiness to Fitzroy Square on the other side of Tottenham Court Road. It is on the opposite side of Gower Street from the world-renowned University College Hospital (UCH), and indeed there is an underground passage under the street to the Anatomy Department of the College. I’ll let readers deduce the purpose of this.  (No, it was not for easier access to the nurses!)

I found one excellent source of information on historic power supplies in London here:

This paper states that ‘even in the late 1960s there were four areas of London that still offered direct current supplies.  … In central London this was not so much a consequence of dilatory action in converting supplies (as alternating current was available everywhere) as the need for direct current for theatres that still used arc lamps and the Fleet Street printing presses, which were heavy direct current users.’ My guess is that UCH still used some DC too.  It isn’t clear where these supplies originated, but there was a DC generating station at Shorts Gardens, just off Shaftsbury Avenue in the middle of theatreland, and indeed directly at the end of the line of Gower Street (although it has become Endell Street at this point).  This was maintained primarily for the use of the theatres, but still used by UCL. Apparently it closed in 1965, so I was only just in time!



If a time-critical event occurs, such as a character arriving at a port, you have to deal with it before the next character arrives. You could deal with this by ‘polling’ the port (checking every few microseconds to see if anything has arrived).  If the computer is busy with, say, a long calculation (here we might mean a few microseconds), or if it is busy polling other ports, by the time it polls, the character could have been replaced by the next one. Disaster! You are losing characters and getting garbage.

Most computers get round this problem by allowing the interface chip to generate an ‘interrupt’, i.e. telling the CPU that something important has happened.  The CPU then has to suspend its current task and deal with the interrupt, hence the name. You have to write some code to tell the CPU how to do this. And this code must be very efficient, otherwise another interrupt might occur and itself interrupt the interrupt.  Soon, you have ‘system thrash’ – it spends all its time switchings tasks and never doing anything useful. Double Disaster!

The reason that real-time interrupt programming is so devilish is that you have to write the interrupt code in low-level (often machine code) instructions for speed, but the slightest error can completely hang the computer.  Then you can’t find out what went wrong!   Each operating system has its own way of handling interrupts.  Remember that after you have handled the interrupt, you have to restore the CPU to its pre-interrupt condition.

Luckily, the Raspberry Pi provides Python functions that let you handle interrupts, but not in real time.