THORNTON FIREDAMP DETECTOR.
Presented by Professor W.M. Thornton.
The detector was fitted to a lamp made by W.E. Gray, London.
CONSTRUCTION.
The Thornton Firedamp Detector was designed so that it could be incorporated into battery operated hand lamps and initially it was used with the Thor lamp and the Gray-Sussman lamp. The success of the detector depended upon the use of suitable material for the detector filament, and many years' research went into the choice of suitable material for this. The filament had to burn sufficiently hot so as to burn the smallest percentage of gases but not hot enough to burn out the filament. Platinum and Tungsten wire were found to be unsuitable. Professor Thornton found that a hard platinum alloy was suitable, when wound into a spiral of such a diameter and pitch that the radiation from one turn could activate the firedamp at the surface of the adjoining turn. He also found that the amount of enclosure was important. By keeping the gauze-covered openings leading to the combustion chamber sufficiently small the filament was able to detect a much greater percentage of firedamp with no risk of burning out.
After the lamps had been tested for safety by the Mines Department Testing Station at Sheffield they received the official approval of the Department.
Murton Colliery was selected by the Divisional Inspector of Mines for the trials of the detector, initially incorporated into Thor lamps. The tests gave good results but the suggestion was made that a side window would be more generally useful than the top window provided in these lamps. This resulted in a new design, involving Gray-Sussman electric lamps, the type used at Murton Colliery, in which the testing element was housed in a chamber attached between the pillars, the back forming a reflector. These lamps proved to be easy to adapt and several were converted for use as gas-testing lamps.
Further tests were carried out using a converted Wolf (Maurice) deputies' inspection lamp. The large space in the top of the lamp proved admirable for housing the testing element, the illumination of the window sectors being very clear and uniform.
In this example the detector was contained in a metal case which was bolted to the top of the battery box and mounted between two of the lamp pillars. The detector consisted of a metal filament within a chamber protected by gauzes, which, on pressing a button, was caused to glow. Any firedamp burnt without flame on the surface of the filament and causes it to glow more brightly. The brightness of this filament was compared with that of a comparison bulb by directing the light from the two filaments upon two half-moon sectors placed side by side to form a circular field of vision but separated by a partition. The brightness of the comparison bulb was adjusted by means of a variable resistance so that at either 11/4% or 21/2% of firedamp, as may be desired, the brightness of the two segments was equal. If no gas was present the left-hand segment was darker than the right but at 11/4% the brightness of the two segments was equal. At 21/2% the left-hand segment was the brighter, and became progressively brighter as the percentage of firedamp increased.
The detector was calibrated to the percentage required by means of a metal tube containing a celluloid disc, one-half of which was shaded to reduce the brightness of the segment lit by the comparison bulb. This tube was inserted in the observation window and the comparison-bulb resistance adjusted until visual equality was reached, the resistance then being locked.
HISTORICAL NOTE.
Professor W. M. Thornton was Professor of Electrical Engineering at the Armstrong College, Newcastle upon Tyne and in 1933 he read a paper before the Institution of Mining Engineers in the Lecture Theatre at Neville Hall, Newcastle upon Tyne, in which he described his detector. This paper - A Miner's Gas-Measuring Electric Hand Lamp - was published in the Transactions of the Institute of Mining Engineers, Vol. LXXXV 1932-1933.