Re: WR overlay designation for a fusible resistor?

On Thu, 24 Apr 2008 07:47:54 GMT, "Arfa Daily" <arfa.daily@xxxxxxxxxxxx> wrote:

:
:"Ross Herbert" <rherber1@xxxxxxxxxxxxxx> wrote in message
:news:oq20141q7aomofm31qpfn4a38qrd5multg@xxxxxxxxxx
:> On Wed, 23 Apr 2008 07:49:17 +0100, "N_Cook" <diverse@xxxxxxxxx> wrote:
:>
:> :W for Wood's metal for the fusible part ?
:> :Googling "wood's metal" AND "fusible resistor" produces nothing
:>
:>
:> Can't say I have ever heard of Wood's Metal being used for fusible
:> resistors in
:> electrical or electronic equipment. I seem to remember that a major use
:> for
:> Wood's Metal was as the fusible element in thermal break-circuit fire
:> detectors
:> used back in the 30's - 50's. They were superseded by much better
:> detectors.
:
:There certainly used to be a pellet of Wood's in the 'daisy' fire
:sprinklers. I don't know of that is still the case. It's a simple, reliable,
:and more to the point 'on-the-spot' and unpowered system. The lid of my
:pressure cooker still has a Wood's metal pressure pressure relief valve,
:which coincidentally, I replaced just last week.
:
:Arfa
:


All of the sprinkler systems I have looked at in the past 50 years or so use the
glass bulb which breaks due to expansion thus allowing the plug sealing the
outlet orifice to pop out under water pressure.

You are correct regarding the pressure safety valve on a pressure cooker. It
does have a slug of low melting point alloy but not sure if this is classified
as Wood's Metal, although it would probably be very similar. Wood's Metal
appears to have set proportions of bismuth (50%), lead (25%), tin (12.5%) and
cadmium (12.5%) designed to melt at approx 150 deg F.

http://evans.mse.berkeley.edu/~dan/msds-evansgroup/msds-Woods%20Metal.pdf
.

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