NEIMME: Lamps
|
THE LIVEING FIRE-DAMP INDICATOR. Click to view image |
This ingenious instrument was devised at the end of the 19th century by
Liveing and is described in the Transactions of The North of England Institute of Mining and Mechanical Engineers 1878-79. Vol. 28, page 167.
The operation is essentially based on the differences of Light evolved from two platinum wires heated respectively in air and the mine gas. The difference in temperature (and hence light evolved) of the two wires is measured photometrically, the spiral G is enclosed in an air-tight metal tube containing pure air, and is therefore, never reached by the gas. The spiral F is surrounded by a cylinder of wire gauze for safety reasons in case an explosive mixture is drawn into the instrument and comes in contact with the heated platinum wire. The atmosphere being tested is drawn through the equipment by suction from the mouth or other means at point E. The incoming air reaches the spiral F through the wire gauze but is completely excluded from the spiral G. The light emitted by the spirals reaches the photometer through glass plates which close the wire gauze tube M and the metal tube N. The sloping surface of the block H (on which the light is reflected) can be moved by the screw K along the graduated scale L until the two inclined faces seem to be equally illuminated. The position of the point of the junction of the two "screens" upon the scale then indicates its position relative to the two platinum spirals and hence the percentage of fire-damp present in the air. There is a narrow glass window C to enable the observer to adjust the block H and read the scale.
In the base of the instrument is a small magneto-electric machine. By turning a handle on the outside of the box which encloses the apparatus an electric current is produced, and this passes through the precisely similar platinum wire spirals, heating them to moderate redness. An increase in the velocity of rotation of the handle increases the electric current in the wires, and correspondingly raises their temperature and increases their brilliancy. The rate of rotation may vary within certain limits, but should be such as to heat the spirals to dull or moderate redness.
When constructed, the scale of the instrument is divided to suit two precisely similar spirals, which in pure air are rendered equally bright by the same current. In its new condition the instrument is capable of giving reasonably accurate results. After being in use a comparatively short time, however, the wire spiral which is exposed to the gas becomes permanently altered in its electrical resistance and thus gives rise to errors. It then becomes necessary to re-set the zero of the scale in air free from gas. The scale can be shifted to a slight extent, and when adjusted, the indications of gas it gives, although not strictly accurate, are sufficiently so for practical purposes. It may not be easy to obtain fresh air for the adjustment if that becomes necessary in the mine and, when the wire spiral undergoes considerable change the adjustment of the zero may not be possible owing to the limited range of motion allowed to the scale. In that case two new spirals must be inserted in place of those in the instrument, and this would cause serious delay.
An objection to the use of the Liveing Indicator in the mine arose from the fact that it furnishes no light, and is so much dead weight to be carried about by the fireman or the other official, in addition to a safety-lamp for lighting purposes. After being in use some time there was difficulty in obtaining a current of sufficient steadiness and constancy to ensure accurate readiness. In unskilled hands the handle may be too rapidly rotated and thus produce a current capable of fusing and rendering useless the exposed platinum spiral. A less rapid rotation of the handle may bring about the same result if a large quantity of gas is present. For these reasons its use was never widely adopted. (Sketch overleaf).
This instrument was presented to Dr. W. Reid by J.P. Hall, Esq. Dr. Reid had it repaired and brought into working order for presentation to the Institute by Dr. D. A. Hall.
October 13th, 1966.