| "Ayerton & Perry's Variable Standard of Self-Induction
As shown ... this instrument consists of two coils wound on sections of concentric spherical surfaces, the inside one of which can be rotated with reference to the outside one, and thus their coefficient of induction varied without changing their resistance. As it is very important that the shape of the coils should not change, the coil forms are made of hard rubber. The scale is divided to read in millihenrys on one side and in degrees on the other. Its range is approximately from 3.5 to 42 millihenrys. Special attention has been given to the connections between the inside and outside coil, and as now made, there are none but soldered joints, and all the connections are exposed to view, so that they can be easily inspected or replaced." From Electrical Measuring Instruments (Leeds & Northrup Co., Philadelphia, 1907) pg 103
This beautiful instrument is in the apparatus collection
at Dartmouth College.