Word of mouth has it that on a pain scale of 1 to 10, the bad tooth that recently made my jaw feel like it had been hit by lightning was a 47. That’s why, far from home and without dental insurance, which only compounded the agony, I needed an emergency root canal. Fortunately, I got one from a great endodontist who once gave himself a root canal and lived to tell about it. My tale of misery began when my left lower molar — tooth number 18 if you are scoring at home — began to be sensitive to cold. Since my mouth produces nothing but hot air, I didn’t see how this could be possible. But sure e…