146 On an Electric-light Fire-damp Indicator. 



connected with one of the poles of the battery B or other 

 source of electricity. Two other platinum wires in the tube 

 run parallel with this for part of the way, as in fig. 2, and 

 each is connected with a lamp. The lamps W and E- are 

 joined, and a branch wire connects them to the other pole of 

 the battery. In fig. 2 the current is represented as flowing 

 through W ; when from diffusion in an atmosphere of fire- 

 damp, the conducting plug Ug is driven up to T l , the current 

 will flow through R, and the red light may then be taken to 

 indicate the presence of fire-damp. 



Fig. 2. Fig. 3. 



R 



p. Porous pot. 

 a and b, desiccating tubes. 



The wires being within the tube, one or other of the lamps 

 must always be shining so long as there is a current, whether 

 the apparatus be in an atmosphere of fire-damp, choke-damp, 

 or air ; and to prevent the mercury being driven out of the 

 tube by too much pressure, bulbs are arranged on either side 

 as in fig. 3, which presents a diagrammatic view of the 

 apparatus. We have found an internal diameter of tubing 

 of about 3 millim. best adapted for ensuring easy mobility of 

 the mercury. The presence of the wires within the tube has 

 interfered with the perfection of the seal ; this we have over- 

 come by the introduction of a little concentrated sulphuric acid, 

 which also serves the purpose of preventing sparking and of 

 lubricating the interior. The use of sulphuric acid necessi- 

 tates the addition of desiccators a and b, fig. 3, to each end 

 of the tube ; but in cases where it has been found advisable 

 not to use sulphuric acid, both the acid and the desiccators 

 have been dispensed with by slightly modifying the arrange- 

 ment of the wires at the lower part of the tube. With this form 

 of apparatus we are readily able to detect the presence of 5 per 

 cent, of coal-gas in a mixture of this gas with air ; and with a 

 mercury seal of less weight and closer proximity of the wires 

 at T and T 1 (fig. 2), it appears possible to get any required 

 degree of sensitiveness. It is proposed to have the apparatus 

 fixed in the main roads and hauling roads in pit installations. 



