Counter for Magnetic Coils

As the eminent British physicist James Clerk Maxwell pointed out in his Treatise on Electricity and Magnetism, it was important for 19th century scientists to count the number of turns of wire laid down in constructing galvanometer coils and similar electrical instruments. To do this, Maxwell used a string attached to a shaft turned by the same lathe that held the wheel on which the wire of the coil was wound. Nails helped count the turns of the shaft marked by the string. A device linked to the wheel measured the wire as it turned on the wheel to form the coil, detecting changes in circumference.

This instrument has a brass wheel 18 cm. in diameter. The wheel turns in a steel yoke with a wooden handle. According to the accession file, it was designed by James Clerk Maxwell for measuring the wire in a coil. It lacks a counting element and has no maker’s marks.

References:

Accession File 218174.

James Clerk Maxwell, A Treatise on Electricity and Magnetism, 2, Oxford: Clarendon Press, 1873, p. 314. Subsequent editions of the book contained the same image.

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