during the funeral of former Prime Minister Margaret Thatcher. August 2015: Maintenance crews discovered the clock to be running seven seconds fast. They removed coins from its pendulum to correct the error, which caused it to run slow for a period. 21 August 2017: Start of a four-year silencing of the chimes during maintenance and repair work to the clock mechanism, and repairs and improvements to the clock tower building. During this time, dials, hands, and lights were removed for restoration, with at least one dial – with its hands driven by an electric motor – left intact, functioning, and visible at any given time. A lift was also installed during this renovation. Bells Great Bell The main bell, officially known as the Great Bell but better known as Big Ben, is the largest bell in the tower and part of the Great Clock of Westminster. It sounds an E-natural. The original bell was a 16 ton (16.3-tonne) hour bell, cast on 6 August 1856 in Stockton-on-Tees by John Warner & Sons. It is thought that the bell was originally to be called Victoria or Royal Victoria in honour of Queen Victoria, but that an MP suggested the bell's current nickname of "Big Ben" during a Parliamentary debate; the comment is not recorded in Hansard. Since the tower was not yet finished, the bell was mounted in New Palace Yard but, during testing, it cracked beyond repair and a replacement had to be made. The bell was recast on 10 April 1858 at the Whitechapel Bell Foundry as a 13.5-ton (13.76-tonne) bell. The second bell was transported from the foundry to the tower on a trolley drawn by sixteen horses, with crowds cheering its progress; it was then pulled up to the Clock Tower's belfry, a feat that took 18 hours. It is tall and diameter. This new bell first chimed in July 1859; in September it too cracked under the hammer. According to the foundry's manager, George Mears, the horologist Denison had used a hammer more than twice the maximum weight specified. For three years Big Ben was taken out of commission and the hours were struck on the lowest of the quarter bells until it was repaired. To make the repair, a square piece of metal was chipped out from the rim around the crack, and the bell given an eighth of a turn so the new hammer