From: hugh@ear.mit.edu (Hugh Secker-Walker)
Subject: Re: Flash Pots
Organization: Massachusetts Institute of Technology
	<1992Apr13.181852.28584@murdoch.acc.Virginia.EDU>
Distribution: usa
Date: Tue, 14 Apr 1992 21:15:04 GMT

In article <1992Apr13.181852.28584@murdoch.acc.Virginia.EDU> brg2x@hagar8.acc.Virginia.EDU (Barry Richard Gribble) writes:

   In article <1992Apr4.024250.19154@usenet.ins.cwru.edu>,
   brown@slc8.INS.CWRU.Edu (Dan Brown) writes:

   |> Does anyone know where to get low amperage fuses of the screwing
   type??? 
   |> 
   |> I have made flash pots from fuses, but this requires a LOW amperage
   fuse...

   |> Dan Brown
   |> brown@ins.cwru.edu

   It may be a little more of a mess to do, and perhaps less safe,
   but it occurs to me that your could use a light-buld instead.
   Certainly a 15W make-up mirror kind of light would do.


When we made some flash pots in high school (1978) we had good results
using detonators made from single strands of wire from household lamp
cord.  By using a large voltage (110), one ensures that the strand
vaporises at exactly the moment you throw the switch, no waiting for a
fuse or lamp to heat up.  We used the base of a 100 watt light bulb as
a reusable holder for the thin wire.  After breaking off all the glass
from the bulb and all the filament supports we were left with two thin
metal posts connected to the base contacts.  We soldered the strand of
wire between these two posts.  Given the corrosion that builds up
after repeated use we found that soldering was the only way of
assuring continuity.  Be sure the two posts are *much* thicker than
the wire strand to ensure that the strand, rather than a post, is what
vaporizes.  The ceramic socket for the lamp-base detonator was mounted
on the side of the flash pot.  Once the bulb-base with wire strand was
installed in the socket, the metal posts could be bent to position the
wire strand in the powder.  As indicated earlier in this thread, if
you use a continuity indicator with the prepped flash pot, *BE SURE*
that the current through the strand is *VERY SMALL* (i.e. a few
milliamps only), as even a small current can heat the strand to the
flash-point temperature.  You can use a larger wire, but then the
mounting posts must be increased in size and thus light-bulb bases are
ruled out.  We used 110 VAC (with no fuse!) to fire the thing, but a
motorcycle battery is more portable.  We used none of the safety
precautions discussed in this thread.... I shudder,... and read on!

For fun, in a dark room, we took some open-shutter pictures of the
detonators themselves vaporising.  The flash and the traces left by
flying molten copper made for some elegant prints!  However, the
prints proved that these detonators generate flying debris in the form
of small, hot blobs of metal!  Thus, although the flash powder burns
completely, the explosion is not clean.  We were able to get the
length of the exposed strand down to about 1/4 inch, which minimized
but did not eliminate the debris.

Really, we amateurs can exchange anecdotes all day :-)
How do professional, reliable, reusable detonators work?

Hugh
--
Hugh Secker-Walker                    INTERNET:  hugh@ear-ache.mit.edu
MIT, Research Lab of Electronics      UUCP:      mit-eddie!mit-athena!hugh
     "The good news is the ear has 30,000 channels to the brain,
      the bad news is they're all nonlinear" -- M.B. Sachs
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