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It is clear that in order to make an improvement in any technical subject it is necessary to have a means of measuring it.  Light is a topic with which William Sugg spent many years experimenting and he realised that it was necessary to establish a unit for the measurement of light.  In 1862, when he had been working on the lighting of the Grand Committee Room of the House of Commons, he decided on an observational experiment.  "I made some pencil markings on a piece of paper and took half a dozen men to read the paper at whatever distance they found convenient.  The answer was one ft candle, i.e. one candle at a distance of one foot.  This provided me with a basis of comparison for all other measurements of light and, indeed, became the unit in common use."

This commentary is drawn from a paper read to the 'Civil's' by Alexander Pelham Trotter in 1892 on 'The distribution and measurement of illumination' which was attended by William.  In the following discussion Mr W.J.Dibden said that Mr Sugg first proposed the ft candle thirty years ago as the unit for illumination.  WTS then added several observations from which the one above is drawn.  He also commented that the room in question was the only room where he had been able to feel satisfied in the lighting. (This important reference was found for me by Geoff Brundrett - a co-member of the CIBSE Heritage Panel who had remembered reading it on some occasion and finally unearthed it! CS)

If the ft candle were to be the unit of measurement, it was necessary to establish the performance of a 'Standard' candle.  This was done by measuring the rate at which the candle - or in this case, a pair of candles - burned using the Sugg's Equal-Beam Candle Balance. The Candle Balance allowed the tester to check the number of 'grains' of wax burnt in a fixed period.
The illustration above is taken from the 1904/8 catalogue and the picture below recently obtained from the Science Museum is clearly the same item. (This picture and the following set are by courtesy of Joan Waters - co-member of the IGEM History Panel.)

The Keats Balance from William Sugg was supplied beautifully fitted in a padded case of which this example exists in the Science Museum

Dr Lethaby worked with William Sugg on a number of scientific products and his early photometer is shown here.   Clearly it was set up on a pair of rails and the label states that the 'Distance Between Lights  60 inches'.

Copyright © Chris Sugg 2006

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