Lightning Rod Insulators (LRIs)
LRIs shouldn't even exist.
In 1879, for example, John Phin noted that if lightning had no trouble passing through a hundred feet of insulating air, two inches of glass was not going to prevent it from spilling over into a house, especially given that the LRI is likely to loose some of its insulating properties in the rain during a thunderstorm. In addition, use of LRIs could create a potentially dangerous Leyden jar. As Phin writes:
"...if a house be furnished with a carefully insulated lightning-rod, and should also have any large surface of metal, such as a tin roof, an extensive system of gutters, or such like, connected with it, it is easy to see that the house must resemble a large Leyden jar, of which the tin roof, or other mass of metal, constitutes one coating, and the lightning-rod and the earth constitute the other, while the insulators and the dry material of the house represent the glass of the jar. If both the outside and inside of this jar (the tin roof and the earth) had been connected together, it would have been impossible to have brought one coating into a condition oposite to that ofthe other. But the rod being carefully insulated from the roof, it is obvious that the inductive action ofthe cloud will bring the roof and earth into opposite conditions; and if a man were to form the path of least resistance between them, the discharge would take place through his body, and he would probably be distroyed (p. 22)."
Examples of LRIs
Other Notes
The German word for Lightning Rod Insulators is "blitzableiterisolator."
References
Phin, John. (1879). Plain Directions for the Construction and Erection of Lightning-Rods. 3rd edition. New York: The Industgrial Publication Co. (Original from the University of California. Found on Google Books).