The police ask members of the public who have seen any suspicious actions near the pylons, which stand in open country, to give them information. Already scrap dealers in the area have been visited, but it is understood they have been unable to help.
Nearly 3,000 yards of the wire have been taken from the line between Markham Main Colliery, at Armthorpe and Bentley Colliery and another 1,000 yards from the section between Yorkshire Main, Edlington and Markham Main.
The thieves had to climb over spikes and barbed wire and then the pylons to unclip the earthing wire.
Then they would have to carry at least 200 yards of the wire, because of the distance between pylons, to the nearest road.
An official of the NCB said 200 yards of the wire would weigh about one hundredweight.
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