Vintage curiosity - What is it and how does it work. - Page 1
Forum section: electronics / general

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 17, 2015, 07:43:14 pm
content: Found this item out in the back lot. It is obviously telephone related from where I found it and the wires attached to it which had been clipped off. I originally just wanted to salvage the many brass washers, nuts and other parts but became curious about what its function was. Does anybody recognize this?There is a hard rubber body. The four brass studs sticking up at the right were the terminal posts which had wire ends attached. The left side of the hard rubber body has a brass plate on the top and has four wells with a brass button at the bottom. Each of the brass buttons is connected to one of the studs, with approximately 5 Megohms of resistance. No measureable connection to each other. The brass plate was connected via a brass screw to a (you guessed it) brass mounting bracket on the bottom.The devices shown were screwed into the wells. All four were filled, I just didn't include all of them in the picture. They are essentially a brass cup with what is apparently an iron based material inner cup with a fingered edge. The fingers retain three internal parts, a carbon post in a ceramic cylinder, a carbon pellet, and a metal pellet, in the order shown. The fingers of the inner cup and the carbon post both appear to contact the brass button at the bottom of the well. There is no give to any of this mounting. The carbon post, carbon pellet and metal pellet all have significantly less than one ohm of resistance.It would seem that this is some sort of lighting protection, or static charge dissipation, but I can't figure out the function of the devices in the wells. Any comments would be appreciated. No real need other than satisfying some curiosity.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 17, 2015, 08:19:35 pm
content: A scale might have helped. The rubber base is about 5 cm by 10 cm, and 3 cm tall.

author: SeanB
author_rank: Super Contributor
timestamp: July 17, 2015, 08:38:30 pm
content: Old phone line protectors, to reduce the damage from lightning strikes near to the line from damaging customer side equipment. The black is a varistor material, which will drop in resistance with high voltage, probably a mixture of zinc oxides and ferrite.Note it does very little in the case of a direct strike on the line, where the wire typically acts like a fuse and blows open for long distances as it absorbs the energy. CPE equipment normally survives unless it has features like lights and memory, where you have a connection via arcing to mains earth.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 17, 2015, 11:17:06 pm
content: Thanks SeanB.Makes sense I guess. Probably when new the resistance between the contact button at the bottom of the well and the terminal stud was much lower than the current 5+ megohms, or is that also some form of varistor that drops in resistance?

author: SeanB
author_rank: Super Contributor
timestamp: July 18, 2015, 07:35:46 am
content: Varistor, probably still in spec if you used 3V, but will drop into the hundreds of ohms range at 400V, and all that is required is it must not leak more than 1mA at 100VAC ringing voltage, so the electric line relay at the exchange( or the electronic equivalent) does not trip into of hook state while ringing, when combined with the 4REN ringing current.Phone equipment is designed to be both reliable and simple to maintain, as it typically will have an operating lifetime in the multiple decade range, and it must have no more than 5 minutes of outage over that period for any reason other than physical cabling damage.