In the gradual spread of lightning conductors through the British colonies of North America, Franklin himself took the leading part. He employed all his leisure time, engrossed though it was more and more by political affairs, in which he was destined to take a world-famous part, in going from one part of the country to another, advocating the use of conductors, advising as to the best mode of their construction, and, whenever he could, examining into the effects of strokes of lightning upon buildings. How minute he was in these inspections, and how practical in the conclusions he almost invariably drew from them, Franklin gives proof in one of his letters addressed to his friend Collinson in London. He tells him that he inspected the church of Newbury, in Massachusetts, which had been struck by lightning, and traced, foot by foot and inch by inch, the road which the electric current had taken, creating great havoc and destruction. ‘The steeple,’ he says, ‘was a square tower of wood, reaching seventy feet up from the ground to the place where the bell hung, over which rose a taper spire, of wood likewise, reaching seventy feet higher, to the vane of the weathercock. Near the bell was fixed an iron hammer to strike the hours; and from the tail of the hammer a wire went down through a small gimlet-hole in the floor the bell stood upon; then horizontally under and near the plastered ceiling of that second floor, till it came to a wall; and then down by the side of this wall to a clock which stood about twenty feet below the bell. The wire was not bigger than a common knitting-needle.’ It surprised Franklin that ‘the lightning passed between the hammer and the clock in this wire, without hurting either of the floors, or having any effect upon them, except making the gimlet-holes, through which the wire passed, a little bigger, and without hurting the wall or any part of the building.’ The inference he drew from this was, that even a comparatively thin mass of metal would give passage to a powerful electric stream. ‘The quantity of lightning that passed through the steeple,’ he informed his correspondent, ‘must have been very great, as shown by its effects on the lofty spire above the bell, and on the square tower below the end of the clock pendulum; and yet, great as this quantity was, it was conducted by a small wire and a clock pendulum, without the least damage to the building as far as they extended.’