[GreenKeys] Starting Up an Old Debate

[WA5CAB at cs.com]

I guess it might have been interpreted that way but I didn't mean to imply 
that the magnetic field strength began to decrease at some point above the knee 
of the core's BH curve as the current continued to increase.  The pulling 
power is proportional to the flux density in the air gap (and several other 
factors).  The incremental change in B for a given change in current is much greater 
when the core is operating below the knee than it would be well above the 
knee where mu tends toward 1.

I suspected that there was some logical reason behind the developement of the 
somewhat more complex holding magnet selector but didn't know what it might 
be.  Thanks for the info.

In a message dated 5/31/2004 12:04:03 PM Central Daylight Time, 
jhhaynes at earthlink.net writes: 
> There's nothing wrong with the saturation idea; we just don't know if
> Teletype used it.  When the core goes into saturation the incremental
> inductance collapses; but the strength of the magnet is no less than
> it was at the onset of saturation.  
> 
> According to Walt Zenner, who got the patent on it, the original reason
> for the holding magnet selector was th reduce the voltage on the cord
> circuits in the TWX switchboards.  With the earlier machines they were
> using 120V on the switchboard cords, where it is exposed to the operators'
> hands and can be deadly.  So the holding magnet selector allowed them
> to run 48V on the cords.  That still seems pretty high, but was no
> higher than the voltage on telephone switchboard cords.

Robert Downs - Houston
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