Electricity is of two kinds, positive and negative. The difference is, I presume, that one comes a little more expensive, but is more durable the other is a cheaper thing, but the moths get into it.

Leacock’s joke turns a scientific distinction (positive vs. negative electricity) into a parody of consumer choice, as if electricity were a fabric or household good sold by quality and price. By pretending to “presume” that one kind is costlier and “more durable” while the cheaper kind attracts moths, he satirizes both the layperson’s hazy grasp of technical concepts and the tendency to translate everything into the language of shopping and thrift. The humor depends on category error: electricity cannot be moth-eaten. The line also fits Leacock’s broader comic method—mock-earnest explanation that exposes the absurdity of overconfident, half-informed reasoning.