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Old 10th May 2018, 3:23 pm   #1
ukcol
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Default "Erinoid" screw.

I was reading the service manual for the Murphy A36 recently when an interesting question arose.

I should point out here that I don't actually own an A36; reading service manuals is part of a general interest I have in the history of domestic radio receiver design and in their circuits in particular.

Anyway, back to that interesting question.

In the alignment instructions for the 36 when adjusting the image frequency rejecter Murphy refer to adjusting an erinoid screw. I have never heard of an erinoid screw or an erinoid anything else for that matter and so I decided to try and find out more.

The only reference to erinoid I could find was in the name of an early plastic material that dates from the beginning of the last century thus pre-dating Bakelite. One of its characteristics is that it is biodegradable! This may not have been important in the 1900s but one of its other characteristics, the fact that it could be produced cheaply, would have been.

So, perhaps my erinoid screw is a screw made out of this material? If so it would make sense for Murphy to call it an erinoid screw just as one might talk of a nylon screw or a brass screw.

If this is indeed the correct explanation for the name does anyone know if erinoid has properties that make it suitable for changing the inductance of a coil or is it simply that the screw was just used to move something else?
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Old 10th May 2018, 3:44 pm   #2
Nickthedentist
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Default Re: "Erinoid" screw.

Seems to have been used for things like buttons, pens and piano keys: https://en.wikipedia.org/wiki/Galalith

The term may have been loosely used to mean any plastic or nonmetallic screw.
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Old 10th May 2018, 4:04 pm   #3
rontech
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Default Re: "Erinoid" screw.

It is possible that they mean a screwdriver to adjust a threaded core in an IF coil. A metal screwdriver would affect the inductance and make accurate setting v difficult.

I remember my father telling me back in the forties that some object I had asked about ( I cannot remember what it was ) was made of " erinoid "
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Old 10th May 2018, 4:43 pm   #4
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Default Re: "Erinoid" screw.

Thanks Nick, yes that's where I got my information.

Hi Ron,

I was referring to the adjustment screw and not the tool.

This is the relevent quote from the 36 manual.

"3. With an insulated screwdriver, adjust the erinoid screw situated in the top of the band pass secondary (or grid) coil can until the ............."
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Old 10th May 2018, 5:02 pm   #5
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Default Re: "Erinoid" screw.

As Nick's link shows, Erinoid is a type of casein plastic.

I guess that it was used as being non-magnetic, and also non-conductive (screwing a brass screw into the coil can reduce its inductance due to eddy currents in the screw). So probably it would have had an iron-dust slug mounted on the end.

No other reason for using it, except that it existed as a suitable material at the time! Milk-derived casein plastics are hardly used now, as they have been superceded by better, mouldable oil-derived plastics. But they did have a few decades of popularity.
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Old 10th May 2018, 8:31 pm   #6
ukcol
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Default Re: "Erinoid" screw.

OK so we are agreed that it is called an erinoid screw because it is made from erinoid. Presumably as erinoid cannot be moulded, manufacturing screws from it would require the screws to be machined.

Thanks for your input gents.
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Old 11th May 2018, 10:52 am   #7
mike_newcomb
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Default Re: "Erinoid" screw.

Hi Colin,
google is your friend in such matters.
e.g. Nick advises of one link.

Googling 'eriniod' produces others of interest, e.g. from the BBC and Graces Guide etc.

Not forgetting google images, where there are many of products and adverts.

Regards - Mike
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Old 11th May 2018, 2:50 pm   #8
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Default Re: "Erinoid" screw.

British Industries Fair 1922.
Attached Thumbnails
Click image for larger version

Name:	eriniod.jpg
Views:	157
Size:	75.6 KB
ID:	162615  
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Old 11th May 2018, 3:33 pm   #9
ukcol
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Default Re: "Erinoid" screw.

Quote:
Originally Posted by mike_newcomb View Post
google is your friend in such matters.
Thanks Mike,

Yes I had already done a google search before I posted this thread. I found several results including the one Nick posted and they all referred to the early plastic except for one about a cricket ground that no longer exists.

The reason I posted was that I felt that concluding that the screw was made from erinoid on the basis that a plastic called erinoid existed may have been jumping to a conclusion. There was still the possibility in my mind that erinoid was a manufacturer or an erinoid screw was a particular design of screw that had a specific function. But in the absence of any evidence to the contrary it looks like it is indeed the material the screw is made from. It may seem the obvious conclusion but that's not how my mind works.

__________________________________________________ ________

Hi Nanozeugma

Thanks for the picture, I have copied it to my computer.
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