Election 2010 Lookahead: Wednesday 21 April

The who, when and where of the campaign.

Labour

Labour candidates Eleanor Tunnicliffe and Brian Tomlinson will take part in a general election hustings chaired by Bamber Gascoigne at Duke St Baptist Church, Richmond (7.30pm). Chancellor Alistair Darling debates with Conservative shadow chancellor George Osborne and Lib Dem finance spokesman Vince Cable on BBC2's The Daily Politics (see below).

Conservatives

Former defence secretary Malcolm Rifkind will present Conservative defence policy ahead of the 6 May general election at the Royal United Services Institute for Defence & Security Studies in London (5.30pm). David Cameron will make an appearance on BBC3's Dermot Meets...programme this evening (see below). Shadow chancellor George Osborne debates with Chancellor Alistair Darling and Lib Dem finance spokesman Vince Cable on BBC2's The Daily Politics' (see below). Conservative candidates Zac Goldsmith and Deborah Thomas will take part in a general election hustings chaired by Bamber Gascoigne at Duke St Baptist Church, Richmond (7.30pm).

Liberal Democrats

Nick Clegg has an early start as he hosts a press conference with Lib Dem spokesperson for children, schools and families at the Work Foundation in London (7.30am). He later joins Lib Dem candidate for Camborne and Redruth Julia Goldsworthy in primary school Q&A on the campaign trail at Redruth Cricket Club, Trewirgie Hill, Cornwall (2.30pm, before making an appearance on BBC3's Dermot Meets... programme this evening (see below). Lib Dem finance spokesman Vince Cable debates with Chancellor Alistair Darling and shadow chancellor George Osborne on BBC2's The Daily Politics (see below). He may also join Lib Dem candidate Susan Kramer in a general election hustings chaired by Bamber Gascoigne at Duke St Baptist Church, Richmond this evening (7.30pm).

The media

BNP leader Nick Griffin spoke on Radio 4's Today programme this morning. In the afternoon, Lib Dem leader Nick Clegg will take part in a special election call edition of Radio 4's The World at One programme, where he will respond to comments and questions from listeners (1pm). Chancellor Alistair Darling, Conservative shadow chancellor George Osborne and Lib Dem finance spokesman Vince Cable will debate economic policy on BBC2's The Daily Politics' (2.15pm), where they will be put through their paces by Andrew Neil and BBC economics editor Stephanie Flanders. David Cameron and Nick Clegg will be interviewed by Dermot O'Leary for BBC3 on the Dermot Meets... programme. The interview with David Cameron will air at 8pm followed by Nick Clegg at 8.30pm. The emphasis will be on questions submitted by first-time voters.

Other parties

Scottish National Party leader and First Minister Alex Salmond will give a speech at the Scottish Trades Union Congress conference in Caird Hall, Dundee today. In Belfast, the Social Democratic and Labour Party from Northern Ireland launches its manifesto at 10am. The nationalist party from Northern Ireland won three Westminster seats in 2005, as they had in 1997 and 2001.

Away from the campaign

A fleet of coaches laid on by the government to bring Britons stranded in Europe by the ash cloud sets out from Madrid today. HMS Albion is expected to arrive in Portsmouth tonight at the earliest, after setting sail from Santander in northern Spain yesterday, to repatriate British troops returned from Afghanistan and around 200 priority British civilians who were stranded on the continent. HMS Ocean and HMS Ark Royal have both been sent to service unspecified ports on the Channel, as thousands of British travellers continue to gather there.

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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.