Vintage Leeds & Northrup Microfarad Capacitor - Page 1
Forum section: electronics / projects

author: MathematicalJ
author_rank: Contributor
timestamp: August 04, 2021, 03:46:47 am
content: I have been enjoying restoring old electrical equipment. I recently acquired a Vintage Leeds & Northrup microfarad capacitor:(Photo album)I opened it up and found that the internals were coated in some kind of wax:The wax clearly is no longer able to perform its original function, so I removed most of it. I have some questions.What was this device used for?How was it used, i.e. what are the operating instructions? Even better, can anyone provide a scan of the original instructions?What is the purpose of the wax?Should I reapply a coating of wax, or can I leave it as it is without affecting the function of the device?Thanks for the help!

author: bob91343
author_rank: Super Contributor
timestamp: August 04, 2021, 04:00:48 am
content: It depends. If the wax is in the electric field, it affects the capacitance. Of course the electric field is everywhere but I am referring to its main location. Is there a nameplate that describes what this is? I can't read the panel.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 04, 2021, 04:44:01 am
content: Intriguing old 'box' ,the capacitors appear to be constructed in layers of plates and dialectic (mica ?) sealed with this wax material so perhaps the wax was used as a barrier for humidity.While you may want to restore it completely, without knowing what the material was its probably best to leave it uncoated and certainly those knife switches will be harder to bring back to life, (reliable contact that is.). Still an interesting device from yesteryear.

author: bdunham7
author_rank: Super Contributor
timestamp: August 04, 2021, 05:11:45 am
content: It is a capacitance test box that can give you selectable capacitances by operating the knife switches. In a way it is similar to a resistance decade box, but this will not be in decades--the capacitors are probably arranged in some binary fashion so that you can get as many different values as possible. There should be markings on the box to indicate the various values. It is quite old, perhaps a hundred years or so. Earlier devices like this had brass pins as switches, the knife style is more modern. Is there a model number readable on it?

author: trobbins
author_rank: Frequent Contributor
timestamp: August 04, 2021, 07:14:18 am
content: One terminal appears to go to a busbar connecting to each of the clamped capacitor segments, via a rolled foil cap. The rolled caps are smaller for the clamped caps with progressively less width. The other terminal probably goes to all the knife switches, with the other end of each knife switch going to the associated clamped end-plate of each cap section. Do you have a DMM with capacitance range?The stability and trimming may depend on the clamping force of each section, and how well it is temp compensated for clamping force. It may be that the meaty metal clamping contributes negligible parasitic capacitance (and just mechanical stability).It would have been a lab based capacitance standard, derived from L&N primary standard. I doubt that there are other photos of such an early beast.The wax was likely to avoid any metallic oxidation, depending on what they have used for plates. Hopefully you haven't changed the tension or structure of anything prior to measuring individual capacitances. Not sure if removing the wax would be a benefit, unless you plan on apply some form of conformal coating, and can measure the before/after change in capacitance.

author: MathematicalJ
author_rank: Contributor
timestamp: August 17, 2021, 02:35:41 am
content: Thanks everyone for the information and advice.Here is a photo album of before, during, and after restoration:https://photos.app.goo.gl/z9nyn5RmcsoahjA59.The model number is 35288. I have never seen this model before. Other similar devices with knife switches have a circuit diagram on the bakelite top plate.My EEVBlog branded BM235 DMM measures the capacitance of each value to be within 4% of the indicated value. Not bad for 100 years old.My guess is that the wax is meant to be protection against moisture.

author: mawyatt
author_rank: Super Contributor
timestamp: August 17, 2021, 02:44:01 pm
content: Quote from: MathematicalJ on August 17, 2021, 02:35:41 amBeautiful restoration, well done!!!Best,