General Radio Type 1491-D precision decade inductor tested in all decades and in working order.
The GenRad 1491 Decade Inductor Box is the most accurate inductor commercially available. The 1491 has a maximum inductance of 11H with a resolution of 1mH.
The decade switch has high-quality ceramic stator and rotor elements and precisely defined ball traps. All contacts are made of a silver alloy and have a positive wiping effect. Shielded toroidal cores are used for small mutual inductance and minimal effect of external fields.
The 1491 inductor decade series are practical elements for use in wave filters, equalizers and rotated circuits throughout the audio and low radio frequency range. As components in oscillators, analyzers, and similar devices, they are particularly useful during the preliminary design period when the ability to vary circuit elements over relatively large ranges is necessary to determine optimal operating values. The 1491 decade inductors have low frequency storage factor Q values that are much larger than those of air coils.