Using a 259 is a big help for antenna builders, but before
using it one should know that the power supply does not
always break down the oxides formed at the circuit
connections, thus showing inconclusive results.
It takes at least 20 volts to breakdown the
oxides every time with confidence ( and that includes silver).
So the use of a wallplug transformer with a 20 volt secondary
to create a spark across the coax connectiori will always
breakdown the oxides prior to using the 259.
Actually, it is always good form to always break down oxides
before using hand held measuring techniques which usually are
battery driven. This is the reason that reed switches are the
contacts of choice when using 12v computor driven circuitry,
since with gold plated contacts the underlying surface
often leaches thru the gold plating.
Have a happy day
Art Unwin KB9MZ....XG (uk)

AI4QJ wrote:

Don't know about a battery option. Art is saying we need 20V to breakover
the oxide resistance. In my short stint as a component engineer years ago,
our research showed that zinc coating could actually take as much as 35V to
break over. Art is entirely correct on the oxide problem. Gold was very low
breakover, however...we had not considered the substrate leaching into the
gold (a very good point) but we considered the gold "flash" coating to be
totally useless and in the 35V region because the gold flash is so thin it
basically rubs off the first time it is used.

Copper easily leaches into and alloys with gold, so when making a gold
contact, the gold is never plated directly onto copper. A nickel barrier
layer is virtually always used between the copper and gold to prevent
this. When properly done, a gold contact will last a very long time.
Gold is the only reliable way to do "dry circuit" (low voltage and
current) switching, but a limited number of good connections can be made
using other metals if sufficient contact pressure and wiping are employed.

Gold flash is much too thin to be a good contact material, as AI4QJ
notes. It's sometimes used on switches designed for "hot circuit"
operation, but not "dry circuit" switches. Its only purpose is to
provide some protection of a base metal surface from corrosion until the
first switch contact is made. After that, wiping and arcing are depended
upon to keep the contact area clean.

If you rummage around in your junk box, you'll probably find miniature
toggle switches rated 250 volts and others rated 20 volts. The former
have silver contacts, for "hot circuit" switching. The latter have gold
contacts for "dry circuit" switching. Some high quality switches have a
good layer of gold (not "flash") over silver. Those can be used for
either "hot" or "dry" circuit switching, but once used for hot circuit
switching, the gold is rapidly eroded, so they can't later be used for
dry circuits.

The design and application of electrical contacts is a discipline in
itself. It's a much more complex and difficult problem than most people
realize.

Roy Lewallen, W7EL

On 26 Jan, 12:42, Roy Lewallen wrote:
AI4QJ wrote:

Don't know about a battery option. Art is saying we need 20V to breakover
the oxide resistance. In my short stint as a component engineer years ago,
our research showed that zinc coating could actually take as much as 35V to
break over. Art is entirely correct on the oxide problem. Gold was very low
breakover, however...we had not considered the substrate leaching into the
gold (a very good point) but we considered the gold "flash" coating to be
totally useless and in the 35V region because the gold flash is so thin it
basically rubs off the first time it is used.

Copper easily leaches into and alloys with gold, so when making a gold
contact, the gold is never plated directly onto copper. A nickel barrier
layer is virtually always used between the copper and gold to prevent
this. When properly done, a gold contact will last a very long time.
Gold is the only reliable way to do "dry circuit" (low voltage and
current) switching, but a limited number of good connections can be made
using other metals if sufficient contact pressure and wiping are employed.

Gold flash is much too thin to be a good contact material, as AI4QJ
notes. It's sometimes used on switches designed for "hot circuit"
operation, but not "dry circuit" switches. Its only purpose is to
provide some protection of a base metal surface from corrosion until the
first switch contact is made. After that, wiping and arcing are depended
upon to keep the contact area clean.

If you rummage around in your junk box, you'll probably find miniature
toggle switches rated 250 volts and others rated 20 volts. The former
have silver contacts, for "hot circuit" switching. The latter have gold
contacts for "dry circuit" switching. Some high quality switches have a
good layer of gold (not "flash") over silver. Those can be used for
either "hot" or "dry" circuit switching, but once used for hot circuit
switching, the gold is rapidly eroded, so they can't later be used for
dry circuits.

The design and application of electrical contacts is a discipline in
itself. It's a much more complex and difficult problem than most people
realize.

Roy Lewallen, W7EL

I worked as a Snr controls engineer( yes I was a trained mechanical
engineer working outside my field) for 20 years at G.E
The bottom line in all contacts is pressure and wiping/scrubbing
action
GE won a suit against it and other wiring companies when the hoo haa
started over mixed aluminum wiring in house holds.
G.E. was dropped out of the cases when it was shown that clamping
pressure
prevented exposure of the contact surfaces. Actually silver contacts
even when normally closed will build an oxide between the contacts.
Because control devices are packaged incardboard boxes for shipment it
is nigh impossibble for the customer to get clean contacts because of
the sulphides exuding from the card board.
Regards
Art Unwin KB9MZ...XG(uk)