Cameron's statement on the riots - live blog

Minute-by-minute coverage of the Prime Minister's statement to the Commons.

Stay tuned for live coverage from 11:30am.

11:33 As you'd expect, the Commons is packed. Cameron should begin his statement shortly.

11:34 Cameron is speaking. It is right that we show a "united front," he says. He pays tribute to Tottenham MP David Lammy for his "powerful words and actions" over the past days.

11:35 Mark Duggan's death was used as an excuse by "opportunistic thugs and gangs", Cameron says. There was no causal link.

11:37 The police made the mistake of treating the issue as one of "public order", rather than crime, Cameron says.

11:38 More than 1,200 people have been arrested, Cameron announces. He repeats his line about "phoney human rights concerns" not getting in the way of publishing CCTV pictures.

11:39 Cameron says it was right not to use the army but promises to look at how they could assist the police in the future.

11:41 Police will have the discretion to "remove face coverings" when they are related to criminal behaviour, Cameron announces.

11:43 The insurance industry is expected to pay out £200m. The government has set up a "high street support scheme" to help businesses recover, and tax payments will be deferred for those in greatest need.

11:44 Ministers will meet the "emergency costs" of those made homeless.

11:45 Cameron moves on to the "deeper causes" of the riots. "This is not about poverty it's about culture," he argues. A first mention for "the broken society".

11:47 The heart of the problem is teenage gangs, Cameron says.

11:48 To the "violent minority", Cameron says: "We will track you down, we will find you, we will punish you."

11:49 Cameron closes by saying that we need to show the world, which has looked on "appalled", the Britain "that doesn't destroy but builds".

11.50: Ed Miliband has taken over to respond to Cameron's statement. Britain wants to return to normality, he says.

11.51: Miliband says he agrees with Cameron on the army -- this is a job for the police. But Miliband asks for clarification for what the army will do to support the police, and how the extra strain on the police will be funded, given that budgets are already stretched.

11.53: Miliband asks: will Cameron now think again about cuts to the police force?

11.54: Can Cameron assure that there won't be an arbitrary cap on the amount needed to rebuild the communities affected?

11.55 "To seek to explain is not to seek to excuse. Why are there people who feel they have everything to gain and nothing to lose from wanton vandalism?" Miliband says that the causes are complex. We can only find solutions through hearing from our communities, or trouble will begin again. How will the government make sure communities are engaged?

11.57 Miliband says we must not forget our own responsbility -- not to the minority of young people who committed the violence, but the law-abiding majority. We must ensure there are opportunities for them and they have the right to expect this.

11.58Cameron is now responding to Miliband. He starts by thanking him and agrees about the need for normality.

11.59: On the police, says Cameron, government's have a responsibility to look ahead to potential problems and contingencies. In the future, they could look at what the army could take over to free up frontline police, in emergencies.

12.01: On police budgets, Cameron says the government is looking for cash reductions that are totally achievable without visible reductions in police numbers.

12.02: The Riot Damages Act does not have a cap on the amount of money available to communities, says Cameron.

12.03 People are responsible for their actions, says Cameron. He reiterates that he hopes this cross-party collaboration can continue.

12:06 David Lammy, the MP for Tottenham is speaking. 45 people have lost their homes in Tottenham, he says. The cry is: "Where were the police?"

12:10 Jack Straw, a former Home Secretary, says Cameron must accept that the cuts will lead to fewer police on the streets. He adds that Cameron must reverse Ken Clarke's prison closures. "We need more prisons," he says.

12:13 David Davis asks what the government will do to prevent "evil-minded people" using the events to increase ethnic conflict. Cameron says the government will work closely with community leaders.

12:19 Nadine Dorries asks Cameron why the police did not have imeediate access to plastic bullets and water cannon. Cameron says that the police came close to using baton rounds but adds that the size and mobility of the crowd meant water cannon were rightly not used.

12:22 We're going to end the live blog here. But stay tuned for live coverage of George Osborne's statement on the economy.

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Inside Big Ben: why the world’s most famous clock will soon lose its bong

Every now and then, even the most famous of clocks need a bit of care.

London is soon going to lose one of its most familiar sounds when the world-famous Big Ben falls silent for repairs. The “bonging” chimes that have marked the passing of time for Londoners since 1859 will fall silent for months beginning in 2017 as part of a three-year £29m conservation project.

Of course, “Big Ben” is the nickname of the Great Bell and the bell itself is not in bad shape – even though it does have a huge crack in it.

The bell weighs nearly 14 tonnes and it cracked in 1859 when it was first bonged with a hammer that was way too heavy.

The crack was never repaired. Instead the bell was rotated one eighth of a turn and a lighter (200kg) hammer was installed. The cracked bell has a characteristic sound which we have all grown to love.

Big Ben strikes. UK Parliament.

Instead, it is the Elizabeth Tower (1859) and the clock mechanism (1854), designed by Denison and Airy, that need attention.

Any building or machine needs regular maintenance – we paint our doors and windows when they need it and we repair or replace our cars quite routinely. It is convenient to choose a day when we’re out of the house to paint the doors, or when we don’t need the car to repair the brakes. But a clock just doesn’t stop – especially not a clock as iconic as the Great Clock at the Palace of Westminster.

Repairs to the tower are long overdue. There is corrosion damage to the cast iron roof and to the belfry structure which keeps the bells in place. There is water damage to the masonry and condensation problems will be addressed, too. There are plumbing and electrical works to be done for a lift to be installed in one of the ventilation shafts, toilet facilities and the fitting of low-energy lighting.

Marvel of engineering

The clock mechanism itself is remarkable. In its 162-year history it has only had one major breakdown. In 1976 the speed regulator for the chimes broke and the mechanism sped up to destruction. The resulting damage took months to repair.

The weights that drive the clock are, like the bells and hammers, unimaginably huge. The “drive train” that keeps the pendulum swinging and that turns the hands is driven by a weight of about 100kg. Two other weights that ring the bells are each over a tonne. If any of these weights falls out of control (as in the 1976 incident), they could do a lot of damage.

The pendulum suspension spring is especially critical because it holds up the huge pendulum bob which weighs 321kg. The swinging pendulum releases the “escapement” every two seconds which then turns the hands on the clock’s four faces. If you look very closely, you will see that the minute hand doesn’t move smoothly but it sits still most of the time, only moving on each tick by 1.5cm.

The pendulum swings back and forth 21,600 times a day. That’s nearly 8m times a year, bending the pendulum spring. Like any metal, it has the potential to suffer from fatigue. The pendulum needs to be lifted out of the clock so that the spring can be closely inspected.

The clock derives its remarkable accuracy in part from the temperature compensation which is built into the construction of the pendulum. This was yet another of John Harrison’s genius ideas (you probably know him from longitude fame). He came up with the solution of using metals of differing temperature expansion coefficient so that the pendulum doesn’t change in length as the temperature changes with the seasons.

In the Westminster clock, the pendulum shaft is made of concentric tubes of steel and zinc. A similar construction is described for the clock in Trinity College Cambridge and near perfect temperature compensation can be achieved. But zinc is a ductile metal and the tube deforms with time under the heavy load of the 321kg pendulum bob. This “creeping” will cause the temperature compensation to jam up and become less effective.

So stopping the clock will also be a good opportunity to dismantle the pendulum completely and to check that the zinc tube is sliding freely. This in itself is a few days' work.

What makes it tick

But the truly clever bit of this clock is the escapement. All clocks have one - it’s what makes the clock tick, quite literally. Denison developed his new gravity escapement especially for the Westminster clock. It decouples the driving force of the falling weight from the periodic force that maintains the motion of the pendulum. To this day, the best tower clocks in England use the gravity escapement leading to remarkable accuracy – better even than that of your quartz crystal wrist watch.

In Denison’s gravity escapement, the “tick” is the impact of the “legs” of the escapement colliding with hardened steel seats. Each collision causes microscopic damage which, accumulated over millions of collisions per year, causes wear and tear affecting the accuracy of the clock. It is impossible to inspect the escapement without stopping the clock. Part of the maintenance proposed during this stoppage is a thorough overhaul of the escapement and the other workings of the clock.

The Westminster clock is a remarkable icon for London and for England. For more than 150 years it has reminded us of each hour, tirelessly. That’s what I love about clocks – they seem to carry on without a fuss. But every now and then even the most famous of clocks need a bit of care. After this period of pampering, “Big Ben” ought to be set for another 100 or so years of trouble-free running.

The Conversation

Hugh Hunt is a Reader in Engineering Dynamics and Vibration at the University of Cambridge.

This article was originally published on The Conversation. Read the original article.