According to Christina Hole’s revision of E. & M.A. Radford’s Encyclopaedia of Superstitions, 1969: 253:

Oaks [Quercus spp.] are thunder trees, once sacred to Thor, and as such they are thought to protect against lightning … The branches, or their acorns, kept in a house protect that house from being struck. When ‘pull-down’ blinds were fashionable, the bobbins at the ends of the blind-cords were usually shaped like acorns for this reason.

This statement, finding supposed ancient survivals in everyday objects, is typical of those made by folklorists at that time. Undoubtedly bobbins, even plastic ones produced today, are often acorn-shaped, but is there any evidence that they are shaped thus to provide protection?

There are early twentieth-century records of acorns being thought to provide protection. In her Discovering the Folkore of Plants, ed. 2, 1996: 109, Margaret Baker (b.1926) records:

In a Sussex cottage, where the writer stayed as a child, oak twigs and oak apples and acorns stood in a spill jar on the mantelpiece, summer and winter, against lightning.

The Cuming Museum in the London borough of Southwark holds a collection of ‘charms’ presented by Edward Lovett (1852-1933) in 1916. This includes a number of cord-knobs or blind-pulls shaped like acorns, umbrella tassels ‘with wooden acorns’, an acorn-shaped brass pendant in the form of an acorn, and a glass perfume bottle pendant also in the form of an acorn, all of which were said to provide protection against lightning (see the Museum’s website).

And, according to an article in the Weekly Telegraph of 5 February 1938, ‘ever since the days of the Druids, the acorn has been the accepted charm against lightning’. As might be expected in a newspaper article, no evidence is provided to support this statement, but useful information on current practices is given:

Quite a number of airmen carry with them when flying an acorn … At an inquest on the body of a farm labourer killed by lightning, a witness testified that it was the worst storm he had ever been out in. ‘But I was not frightened’ he added, ‘I had acorns in my pocket.’ Not only airmen but others carry this charm among them steeplejacks. Many steeplejacks would not dream of going aloft in stormy weather without carrying an acorn.

Thus it appears that in the first half of the twentieth century acorns and acorn-shaped objects were believed to provide protection from lightning, but there is no evidence to suggest that this belief is a survival of ancient Scandinavian or Druid beliefs.

Images:  main, acorn, fruit of pedunculate oak (Quercus robur), Old Deer Park, London Borough of Richmond upon Thames; inset, Geffrye Museum, London Borough of Hackney; both August 2015.
Thanks to Tony Smith who drew our attention to the Weekly Telegraph article.