General Radio GR-107N variable inductor

Beautifull variable inductor

A suprice package from Ken, a fellow GR collector from the USA who is in the happy circumstance to live nearby the MIT to visit the regular flea-market that is there. My wish still was a GR standard inductor and to my supprise today a package arrived with a piece of paper stating “Hi Fred, enjoy,  Ken”. And that I’m sure doing.
Thanks Ken, it is beautifull.

The label

This one is the N-version, Interesting is they are made from somewhere in the early 30’s upto at least end 60’s because my 1968 catalogue has it listed. That shows a black dial, the 1936 picture shows a aluminium dial but other knobs. Befor 1936 they used the 0-100 dial, that was not direct reading. So mine must be form before 1936. The serial mumber is 150 so probably early 30’s. According the specs in the 1950 catalogue the N is:

Ls=90 to 500 mH but mine is 48.3 to 561mH at 1 KHz and 2.5% of max mutual inductance . Seperate coils are 1% full scale. Parallel connection is 1/4th of series inductance +/- 0.1%
Frequency error is (increase in inductance at higher frequency ) f² / f²0
fo= 1/2piSQRT(LC0) So at 561mH and 34pF this is 36441 Hz . For instance measured at 5KHz it then should  be=0.018826 +0.561= 579.825mH
Mutual inductance 0-110mH
Typical C0:  Cs=34pF and Cp=41pF
DC resistance 440 Ohm, mine is 656 Ohm
Max: 0,18A and 15W mine is 0.14A
Codeword Hover

Z at 1KHz is (656+j3524.8) Ohm so Q= 5.37 or D=0.186 but at 100KHz it is Q=573 or D= 0.00186

The 1936 version

Mine is different on several points. The Type 107 is made out of two coils. A rotor and a stator concentrically mounted. Turning the rotor changes the coupling and by so the inductance (like a variometer). The coils are made out of stranded wire with the seperate strands insulated from each other. After that the coils are baked in a high melting point material. The are mounted in a phenolic panel.

The scale is direct reading in Ls on later models (post 1936) .  Lp is 0.25 x Ls
The 4 terminals are to connect both coils in series or parallel when used as a single fixed inductor or seperately used as a mutual inductance.

1968 catalog

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