Joule’s Spiders

Before my first visit to the Science Museum’s stores, I’d imagined having to search for my mysterious magnetic instruments in the midst of much dust and cobwebs in the warehouse from the closing scenes of Citizen Kane.

In the rather more ordered and hermetically sealed rooms of Blythe House, the spider threads I found were of a much cosier sort. Encased in their own tiny frame, they rather reminded me of my great-grandparents in their wedding portrait.

The two cocoons of Diadema spider silk are surviving samples of the types used in the dip circle designed by James Prescott Joule. (Yes, that Joule.)

As scientists became more interested in magnetic phenomena in the late eighteenth century, more effort was made to improve the apparatus used in their study. The friction of pivoted needles found in many magnetic instruments was a problem limiting the accuracy and ease of making measurements.

The usual method involved the needle’s cylindrical axle rolling on agate planes as it aligned itself with the surrounding magnetic field.

Joule turned to spider threads to create an alternative suspension method. JD Chorlton examined one of Joule’s dip circles after the latter’s death, describing it as follows:

“The needle, constructed of a thin ribbon of annealed steel, weighing 20 grains, is furnished with an axis made of a wire of standard gold. This axis is supported by thread of the Diadema Spider attached to the arms of a balance suspended by a fine stretched wire. The whole is hung by a wire which can be twisted at the head through 180°”.

Strong, resilient and light, spider silk sounds like an ideal material. In practise the silk was too fiddly, and the needle’s weight and friction with the axle meant that the thread would be prone to snap. Still – it’s a lovely story which gives a sense of the patience, precision and ingenuity required of scientific investigation.