For more than 50 years election night audiences have been fascinated by the swingometer, which shows the percentage change in votes from one party to another. It was made famous by Robert McKenzie.
The first swingometer appeared in 1955 and was created by Peter Milne to explain some results in the south. It was refined by Robert McKenzie and David Butler making its first national debut (shown here) in 1959.
A swing of one percent amounts to one voter in every 100 switching their vote from one party to another. The 1964 swingometer linked the size of swing to a specific result for the first time.
The swingometer also showed how a relatively small number of voters could change an election outcome. It was still centre stage for the 1966 election despite a new reliance on computers.
The swingometer went into colour for the 1970 election. When some swings to the Conservatives were higher than expected an extra section had to be painted on.
In February 1974 the swing principle was demonstrated with this wall chart. Moving the slider from right to left would show which seats would fall to the other party on a given swing.
There was also a table top model for the February 1974 election which produced a hung parliament shown by the yellow shaded area on the swingometer.
By 1979 computers had taken over the number crunching on election night and the old cardboard swingometer was starting to look dated.
But the 1979 election was still a triumph for the old device. Robert McKenzie was able to boast it had predicted the result faster than the BBC's computers and at a fraction of the cost.
The rise of the Liberal/SDP Alliance in the 1980s led to the swingometer being mothballed since it could only show a swing between two parties, not three. Computers made the predictions, and Peter Snow presented.
The swingometer returned for the 1992 election and went hi-tech with lots of new features. It showed where in the country MPs were vulnerable and pushing the pendulum turned blue MPs red and red MPs blue.
The 1997 swingometer travelled further than any of its predecessors - it had to cope with a swing of 10.2 percent from Conservative to Labour. The new swingometer relied entirely on virtual reality graphics.
Peter Snow was not the only person able to enjoy playing with a swingometer in the 2001 election, which had a 3D appearance. An online version was available for all to enjoy on the BBC News website.
Peter Snow decided to hand over the pendulum after the 2005 election after more than 20 years as an election graphics guide for the BBC.
Jeremy Vine has taken over from Peter Snow for the 2010 election. The swingometer will use the very latest in computer graphics. You can watch videos of the swingometers in action at www.bbc.co.uk/archive.