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South Western Electricity Historical Societywww.swehs.co.ukElectric Arc Lamps in Bristolby Peter LambIn looking at old photographs of late Victorian or early Edwardian scenes, many of you may have noticed very, decorative street lights gracing the foregrounds. These lamp standards had long cylindrical shapes above the lamp, which distinguished them as being electric arc lamps. You may have wondered, like me, what was inside these housings. These cylinders, known colloquially at the turn of the century as "chimneys" were not chimneys at all, but housed the complex mechanisms regulating the carbon electrodes. Only two lamp standards of this distinctive design remain as street furniture on the Bristol streets and these are situated at The Mall, Clifton Village. Public lighting in Bristol prior to 1893 was by gas using a simple gas jet, which only provided a light of 15 candle-power. The gas mantle, at eight times brighter, had not been invented prior to this time, and a commercially viable design did not become available until 1895. A public electricity supply commenced in Bristol in August 1893 when the first generating station was commissioned at Temple Back. At that time, generation was solely for domestic and public lighting and there were two systems installed alternating current (AC) and direct current(DC). The AC supplies were used primarily for the domestic supplies, distributing at high voltage (2,000volts, single phase) and transforming down to 105/210volts at small substations. For lighting in domestic dwellings, incandescent filament lamps, with carbon coils as filaments, were used. Arc lamps were not used in the home, since they were too fierce and too smelly. The incandescent lamp, which had first appeared in the mid 1880's, had a luminosity of 15-100 candle-power being brighter than the gas jet. The great advantage of the arc lamp at 1500-2000c.p was its extreme brightness at 100 times greater than the gas jet and also greater than the incandescent filament lamp. Therefore it may be appreciated why it was more suitable for street lighting. The DC supply generated at Temple Back was at 600volts for public lighting using 96 arc lamps, initially connecting 12 per circuit in series, thus giving a voltage per lamp of 50volts. Over a decade later the voltage was changed to 500volts with 10 lamps per circuit. Before describing the Bristol arc lamps, I would like to take you through a brief history of the development of the arc lamp prior to 1893. Sir Humphrey Davey is credited with inventing the first arc lamp, when he demonstrated his invention at the Royal Institution in 1810. It was powered by batteries and used charcoal elements enclosed in a vacuum. The vacuum allowed a longer arc with a much higher voltage. It was some years later (1844) that the principle was further developed by a Frenchman by the name of Foucault. He used carbons from the retorts of a gasworks, which were more durable. Thomas Wright devised the first arc lamp, which involved adjustment of the carbons automatically as they burnt away, and W.C.Staite used an electric current for the regulation of the carbons. Foucault responded in 1858 by producing his regulating lamp. In 1876 Paul Jablochkoff, a Russian living in Paris, produced what was considered at the time to be a big breakthrough, selling 4000 within a few years. His device was known as the "electric candle", since it involved two parallel carbon rods in an upright position separated from each other by a layer of Piaster of Paris. As the carbons were consumed, the positive rod was eaten away at twice the speed of the negative rock so alternating current was preferred to equalise the erosion. In other later arc lamps the positive carbon rod was made twice the size as the negative rod. Jablochkoff's invention was soon overtaken by lamps with more sophisticated regulating devices and there were many to choose from by 1893. Arc lamps were devised to work on both DC and AC, but a brighter light was achieved on a DC system. Also some lamps were open and some were enclosed. There were advantages and disadvantages with enclosing the arc in a vacuum or inert gas. The carbon rods did not bum away so quickly when enclosed, but soiled the glass enclosure, thus requiring regular albeit different maintenance. This was partially overcome by using carbons with a purer content i.e. less metal oxides included. Not only was there a wide choice of manufacturers, but also a wide choice of regulating devices, which can be condensed into five varieties as follows :-
The Brockie-Pell lamps are described in some technical literature as being of the type with two solenoids and a brake wheel. Professor Silvanus P.Thompson describes the Brockie Pell Lamp as having a see-saw leverage system First of all, it is necessary to explain the basic principle of most arc lamp regulating devices, which involved two solenoids, since the majority of regulating lamps consisted of these basic parts. The diagram illustration no.2 shows the Pilsen Lamp with two solenoids. The basic principle of this lamp was for the main coil M acting on a bi-conical core C to pull one carbon apart from the fixed one, thus striking the arc. When a current is drawn through the shunt coil S, it pulls the carbon rod in the opposite direction, until a balance is achieved. As the carbons burn away they are automatically moved closer together maintaining a specific gap. Although I am unsure, which specific type of Brockie-Pell arc lamp was used in Bristol. A patent design was filed in October 1881 by J.Brockie using two pairs of carbons and this is shown in the diagram illustration no.3, which shows the complex mechanisms involved. It is not intended to describe in extreme detail the workings of this arc lamp, suffice it to say that they were designed to achieve two objects :-
By 1911 there were 695 arc lamps connected to the Temple Back DC system involving 5 different types of arc lamp and three different manufacturers. All the arc lamps had two pairs of carbons with added mechanisms as above, which changed over the carbons to a second pair of carbons. One pair of carbons only lasted 6-8 hours and one pair would barely last one night before requiring the replacement of the carbon rods or "pencils" as they were known locally. Also the men who changed the carbons were known as "trimmers", a relic from the days of trimming oil lamp wicks passed on to the gas men trimming gas mantles and thence to electricity workers! The lamps on the system in 1911 were :-
(David Gledhill, member, is thanked for some of the information contained in this article) |