272 TRANSACTIONS OF SECTION G. 



This is a form ot cascade amplifier with resistance coupling. An even num- 

 ber oi valves must be employed, and back coupling from the last to the first 

 produces the required trigger action. One of the forms described is seen in 

 figure I. When the grid Gi becomes positive, say, on account of the arrivai 

 of an electric stimulus, the anode current through r^ becomes greater, and the 

 potential of the second grid falls. In consequence the anode current through 

 r^ decreases, and therefore the potential of Gi, to which ri is connected, becomes 

 more positive, --is there is no restoring influence, the anode current through 

 the instrument / increases to the highest capacity of the tube and battery. 



6. Gaseous Ignition by Hot Wires. By Professor W. M. Thornton, 



D.Sc, D.Eng.^ 



This question is important where the possibility exists of inflammable gases 

 coming into contact with incandescent wires, in coal mines or submarines, for 

 example. From an examination of the behaviour of wires of platinum, nickel, 

 iron, tungsten, molybdenum, gold, and silver heated by electric current in mix- 

 tures with air of hydrogen, methane, ethane, pentane, ethylene, methyl and 

 ethyl alcohol, ethyl ether, benzene, coal gas and petrol, the following conclusions 

 were reached : 



1. The least igniting current is a linear function of the diameter of the wire. 



2. Ignition is for a given size of wire independent of the calorific value of 

 the gas. 



3. It is independent of gas pressure down to a limit of about 10 cm. Hg, 

 when it suddenly fails. Above atmospheric pressure it rises slightly. A 

 platinum wire -02 cm. diameter ignites hydrogen (30 per cent.) at 5-4.5 amperes 

 at atmospheric pressure/ and at 6-0 amperes at 100 lb. per sq. inch. 



4. Gaseous combination, which proceeds automatically to explosion, begins, 

 in the case of platinum wires, with the wire at about 200 deg. C, well below 

 red heat. 



5. With an explosion vessel of 50 c.c. volume it is practically impossible 

 to ignite methane by hot wires. Platinum wires glow white-hot and melt, but 

 do not ignite the gas, which, on being tested afterwards with a spark discharge, 

 immediately explodes. 



6. Electric and magnetic fields have no direct influence on hot-wire ignition. 



7. The temperature of platinum wires glowing brightly by surface com- 

 bustion is, when measured by its change of electrical resistance, much lower 

 than the apparent optical temperature. 



8. Ignition is traced to an action occurring, if not within the surface layer 

 of the metal itself, so close to it that the ordinary gas laws do not come into 

 action. It is inferred that the mechanism of hot-wire ignition is an attack 

 upon oxygen either within the wire or by positive ions of combustible gaa 

 ejected from it. 



In the afternoon a Sectional Visit was paid to the Eoyal Naval Cordite 

 Factory, Halton Heath. 



SATUEDAiT, SEPTEMBER 13. 

 'I'he following Papers were read : — 



1. Submarine Mining. By Commander A. L. Gwynne.^ 



* See Phil. Mag., 38, p. 613, 1919. 

 1 See Engineering, Sept. Ij9, 1919, p. 389. 



