470 SECTIONAL TRANSACTIONS.—G. 
sparking freshly cleaned poles the same number of times in pure hydrogen, 
ignition of the 35 per cent. mixture could not be obtained with less than 
6.0 amperes, giving a large thick arc. It is clear that this kind of ignition also 
does not depend upon the energy of the arc, which is 225 times greater in the 
latter case. Heating the metal in hydrogen to arc temperature destroys that 
which causes ignition of a subsequent explosive mixture. The direct and only 
chemical effect of heating iron in pure hydrogen is to reduce an oxide film 
or to burn out oxygen absorbed in the surface. In cooling, hydrogen may be 
absorbed; if so, its presence greatly retards ignition, though it facilitates 
electron discharge. 
The evidence is that ignition does not depend upon the heat energy of the 
arc, nor upon the total electron discharge, which is greater since the current 
is so much larger, but upon the emission of that which is suppressed by the 
hydrogen. This can only consist of oxygen normally absorbed or absorbed by 
the poles in contact with air and activated by the combined effect of the 
thermionic discharge and strong electric field at break. 
24. Prof. W. M. Tuornton.—A Safe Method of Lighting Coal Mines. 
The remarkable increase in the number of cases of nystagmus amongst coal- 
miners, known to be produced by imperfect lighting at the coal-face, calls for 
improved methods of illumination. The direction in which this has been 
hitherto sought is by the use of more powerful portable low-voltage electric 
lamps. The maximum voltage permitted at the face for signalling purposes 
is 25, direct or alternating, and with this unarmoured cab-tyre flexible cables 
may be used. An evident method of improving illumination is to use 25 volts 
with fixed or portable lamps. The objections raised to any such scheme are 
solely those of ignition, since shock is impossible at such voltages and the 
circuits are non-inductive. 
Recent observations on the influence of frequency on the ignition of methane 
(fire-damp) show that by the use of much ‘higher frequencies than those now 
employed for power purposes a higher factor of safety can be obtained. For 
example, ignition of the most explosive mixture of fire-damp in air—9.5 per 
cent.—by the break of a 200-volt direct-current circuit occurs at 0.5 amperes. 
At a frequency of 50 the least igniting current is 6.5 amperes, while at 160 it 
rises to 23 amperes. At a voltage of 20 and a frequency of 160 it is impossible 
to ignite this mixture with a clean break of less than 150 amperes. Now the 
currents taken by 15-watt gas-filled lamps suitable for use at the face do not 
exceed 1 ampere. There is, therefore, a wide margin of safety possible. 
A system now being tested consists of a 160 frequency supply at 15 volts, 
each circuit having about twenty portable lamps. These have double filaments ; the 
shorter of the two, connected to the battery contained in the lamp, gives 2 candle- 
power, by which the miner travels. ‘Ihe larger one carries 15 volts, by which, 
when connected to a plug housed in a heavy movable block, the illumination at 
the working face can be raised to 15 or more candle-power, the shorter filament 
being then cut out. In the case of failure of supply, work can be continued on 
the lamp battery. Each plug has a special interlocking contact by which a 
break of circuit is prevented until the lamp is switched over on to the portable ~ 
cell. ‘There is, therefore, no risk of open sparking. The plug-box also contains — 
a fuse, by which, in the event of a short circuit in a lamp, that alone is cut 
out. The less likely case of a short circuit on the main heavily cab-tyre flexible 
cable which runs through all the plugs is provided for by a choking coil per- 
manently in circuit, which limits the current to less than that which could 
cause ignition of a 9.5 per cent. mixture if broken in the open. The system is 
therefore protected by all the usual devices against excess current, but its 
chief safety is in the use of currents of such frequencies that, without any 
other change, the factor of safety is raised to a high value. 
25. Mr. J. Scorr-Taccart.—Developments in Wireless Reception. 
Particulars are given of various developments in wireless reception, these — 
being of an original character. The application of the methods to the recep- 
tion of continuous waves, spark signals, and telephony is discussed, and circuits 
particularly suitable for the reception of broadcasting are described. Attention 
is given to the provision of stable and reliable dual amplification circuits for 
wireless reception. 
