Artificial heat:

A set of thirty coionies of honeybees was divided into five groups, four of which received different types of artificial heating during the winter of 1940-1. The general effect of hating the colonies appeared in these preliminary experiments to be to reduce the area of brood present in mid-April and to increase the winter consumption of stores, in direct contradiction of any belief in a beneficial effect on the colony. Colonies continuously heated throughout the winter were a little better than the controls in both respects, but their advantage ws not significant, whereas there was little doubt of the adverse effects of the other heating treatments.

Butler, C. G. and Cockbill, G. F. 1942. Preliminary investigations on the value of electric heating of beehives. Annals of Applied Biology

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