Set Of Six Igranic Honeycomb Duolateral Coils. TRF Coils From The 1930s

This is a Set Of Six Igranic Honeycomb Duolateral Coils. TRF Coils From The 1930s presenting a real slice of radio history.

The honeycomb Tuned Radio Frequency (TRF) coils have the following ratings:-

1. 300
2. 150
3. 75
4. 50
5. 40
6. 35

They date to the 1930s and are all in pretty good condition.

They are all stamped with Patent numbers and registered numbers. The key De Forest patent is 141344, dating to the 1920s, and filed by Forest Radio Telephone and Teleg, as mentioned in the advert below: 

Igranic Electric Company Limited, were based at 147 Queen Victoria Street, London, EC4 and the Igranic Works, Elstow Road, Bedford. They were manufacturers of electrical componentry and radio accessories until 1961.

Extract from the Patent 141344:

A coil is wound with the superimposed turns crossing one another at an angle and spaced equidistantly to form a cellular structure with air spaces between the coils. The coils may be separated by layers of insulating-material such as blotting or other paper, cardboard &c., which may be perforated. Alternatively wooden or other pins may be used. After winding, the coil is coated with a binding-substance. This construction is stated to reduce the distributed capacity and high-frequency resistance.

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