Excerpt from Construction and Calculation of Absolute Standards of Inductance
The axial length of the windings was measured by means of a Fuess cathetometer fitted with a micrometer microscope, the cylinder being placed on end. The cathetometer was movable about its own axis ... and a standard meter bar of the Société Genevoise was placed so that by a slight turning of the axis the bar was brought into the field of view. To make sure that any differences in focus might not affect the measurements the micrometer was calibrated at every reading.
As the last turns of wire were so close to the guiding rims that they could not conveniently be observed, readings were taken at noted wires near the ends and also at the windings between the sep arate coils into which the total coil was divided, settings being made on a fine German silver wire laid in the grooves between adjacent wires. The total number of turns being known, the total length of wire could easily be calculated. The readings were taken along six teen different generators of the cylinder. The lengths of the sub divisions were determined with about the same absolute deviation as the total length. The wire was calipered with a Brown Sharpe wire gauge while being wound on the cylinder. The insulation being hard enamel it was assumed not to be compressed by the ten sion of the wire against the marble. The diameter of the wire, as calculated from the length measurements, came out smaller than the gauge measurements, which means that the wire stretched slightly while being laid on. The computed value of the diameter of the wire is used in the calculations.
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