The British decimal fifty pence (50p) coin – often pronounced fifty pee – is a unit of currency equaling one half of a pound sterling. It is a seven-sided coin formed as an
equilateral-curve heptagon, or Reuleaux polygon, a curve of constant width, meaning that the diameter is constant across any bisection. Its obverse has featured the profile of Queen Elizabeth II since the coin’s introduction in 1969. Four different portraits of the Queen have been used, with the latest design by Jody Clark being introduced in 2015. The second and current reverse, featuring a segment of the Royal Shield, was introduced in 2008.
Twenty pence and fifty pence coins are legal tender only up to the sum of £10; this means that it is permissible to refuse payment of sums greater than this amount in 20p and 50p coins in order to settle a debt.
As of March 2014 there were an estimated 948 million 50p coins in circulation
In 1967 the Deputy Master of the Royal Mint approached the Decimal Currency Board to ask for their advice on the introduction of a new coin. The 10 shilling note then in use was lasting only five months and it had been suggested that a coin, which could last fifty years, would be more economical. The problem with this was that all coins are arranged in “tiers”, each coin in a tier having the same weight-to-value ratio so that a bag of mixed coins could be weighed to ascertain the value so long as they were all bronze, all silver, etc. Each coin was identified within its tier by its size and each tier had to be capable of being identified by sight and touch. This was achieved in the then existing sets by the use of different materials (“bronze”, “brass” and “silver”) with the bronze coins having plain rims, the nickel-brass threepenny bit being 12-sided and the silver coins having milled rims. If the 10-shilling coin was to be made in the same tier as the silver coins it would have to be twice the weight of the Crown (then and now only in use for commemorative pieces) and it was generally agreed that that would make it very unpopular and expensive. It would therefore have to be in a new tier of its own.
The Mint could not find a suitable metal which was sufficiently different in colour to the existing coins and which would not tarnish. This last point was thought to be important because the new coin would be the most valuable coin in general circulation in the world (about £8.40 in today’s values). It therefore had to be a different shape; various methods had been used overseas to overcome this problem but none were without drawbacks. A hole through the coin did unacceptable things to the Queen’s head (which is a legal requirement on British coins), and wavy-edged, flat-edged or square coins could not be used in the coin-handling machinery which was then coming into increasing use in industry, banking and vending. To be used in a vending or sorting machine a coin would have to roll under gravity and be capable of being measured without being presented in a special way, in other words it needed a constant breadth at whichever angle it was measured.
The Technical Member (and the only engineer) on the Decimal Currency Board was Hugh Conway, at that time President of the Institution of Mechanical Engineers and Managing Director of Bristol Siddeley Engines, Bristol. He had found in a mathematical textbook a formula for a non-circular shape of constant breadth and asked the design office at Patchway, near Bristol, which normally worked on the engines for aircraft such as Concorde, Vulcan and Harrier to draw out the shape. However, this turned out to be a wavy-edged form with re-entrant sides which would not roll and which could not be measured easily. A designer, Colin Lewis, suggested a much simpler shape which in its basic form is an equilateral triangle with a small circle centred on each apex and with a larger circular arc centred on each apex but tangential to each of the two opposite small circles. Wherever it was measured, the breadth of this shape was one small radius plus one large radius. (The small radius was not strictly necessary to the geometry, but made the shape more practical by removing inconvenient sharp points and reducing the rate of wear, and therefore change of size, in handling). The number of corners could be any odd number greater than one. A drawing was made to illustrate the proposal which was accepted by Hugh Conway. He chose seven sides as a compromise between too radical a shape, which might not be acceptable to the public, and having too many sides, which would make a shape visually difficult to differentiate from a circle. The shape was drawn out by Dave Brown and samples made from stainless steel by the Model Shop, together with a section of perspex channel with a bend to demonstrate that the “coin” would roll around corners and drop through gauging slots. The legend “50” was photo-etched (from a master drawn by Ray Gooding) on the faces of the samples since it had already been decided that the new coin would be the first coin of the new Decimal series.
by simon schofield