Polarity relay



Page initiated on 26 November 2013

Status: 27 November 2013



This page is setup because we encounter ongoing problems with the polarity relay operated in conjunction to our Siemens Geheimschreiber repair project; especially after some demonstration, where the system ran each time for some minutes.

Last Saturday 23 November, during demonstrations the system started to fail again. What might causing this time and again?


Some years ago, I had a relay box with the red printed notice on it: that the relays should be stored in a special way to one another. This is curious, as relay normally do not need such special provision. Why?

The only reasons may have been that the magnetic field around it should be kept intact and not being loaded too much preventing magnetic leakage.

Peter Eijlander, who viewed the failing demonstration, took his smart phone and showed with the compass function that these relay indeed have magnetic fields around it.

In the past, during relay alignments I saw already that between both poles A and B (drawing shown below) sometimes polarised metal chips were found; just following the magnetic flux lines.

Why, and how does this match with the outside ring which I have given randomly both N and S poles? Whether it should be otherwise around I do not yet know, but should be one of the aims of this reflection.

It is in physics considered that the magnetic lines outside a magnet moves from North towards the South pole; inside, of course, it flows from South to the North pole.   


All our projects are aiming to enhance the understanding of a phenomenon, so will it be this time too.

Please notice, that experiments may sometimes causing pitfalls; these are inherent to experiments without the right information at hand.


How should we approach the task first?

My first approach is using a simple compass. It allows determining between the North and South pole. In some respect strength can be compared as we bring the compass at some distance. The Siemens T 64 relay series is the successor of the pre-war type T43a can be used as to comparing values and deducing principle means.

Let us thus taking both the failing T43a and some well performing T64 relay in consideration.


Showing my impression of the pre-war Siemens type T43a. I did it from memory, thus some perspective may deviate

(partially faulty, please consider the next drawing and explications)

The contacts T and Z represent respectively: Space (-) and Mark (+)

The switching strip between the contacts T and Z and at the other end in between the to be be magnetised poles A end B is most likely consisting of 'soft iron'. Soft iron magnetise very easily and looses it quite instantly when the magnetic flux stops; only minor remanence remaining.


What is apparent, is the fact that the core of the coil driven relay section is in some way kept parallel onto the magnetic outside magnetic ring. I call it a ring as it forms a magnetically closed system.


On 27 November 2013

It proved that some of my former judgements are in some respect invalid.



Shown is the rear side of the T43a relay

The grey colour of both magnetic cores (outside ring) indicate that it originate from production later in the war. 



The previous photo shown a bit more in detail

This particular detail is not visible when we consider the earlier types



Closer investigation brought discrepancies to light, which is being revised

Owing to this the magnetic poles designations necessitated also correction.

Looking over the consequences, the principle on which such a polarity relay relies needs also a bit different approach.



Shown a warning that these relays types should be kept packet in this box. Indicating that otherwise the relay parameter will deteriorate


Because the inner box is designed such that the relay can only be stored in a fixed position; versus the magnetic field surrounding the relay device.


This is nevertheless, not the box I had in mind, but it might indicate similarly. Shown is a modern type of post war years (probably type T 64)

These relay type has a far more distinguished relay hold at the T or Z contacts.


To be continued in due course


By Arthur O. Bauer