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INVENTORS
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Reports on the Dangers of Electrically Operated Signalling in Mines
Home Office
Whitehall  S.W.
6th. Oct., 1916
From The Chief Inspector of Mines,
Home Office,
London, S.W.  
No. to quote 25,027/15. Gentlemen,
I am directed by the Secretary of State to call your attention to the important report enclosed herewith, of a further investigation on Electric Signalling in the Mines with Bare Wires, which has been carried out at the Home Office Experimental Station at Eskmeals by Dr. Wheeler and Professor Thornton.
The present report confirms and amplifies Dr. Wheeler's previous Report published last year, to which the attention of Mine owners was called by my circular letter of 19th. April 1915.
It emphasises again the fact that with the usual type of bell and magnitude of Battery at present employed,  nearly every break-flash that occurs when the bare signal Wires are separated would ignite a mixture of firedamp and air, containing between 7.5 and 9,5 per cent of methane were such a mixture  to surround the wires at the  time.
Relays, although not requiring for their action so great a Battery Power as Bells, have, owing to their lower inductance, a lower “igniting current” at which the break-flash on the Signal Wires becomes dangerous.