This example,
kindly donated by Donald Thomson is the Mullard ZM1175, which
uses a wire ended base. The valve has a set of numbers arranged
in front of one another and uses the neon glow effect to illuminate
one of the digits. They use an HT voltage to strike the neon and
this is applied to the particular digit to be displayed. Usually
the colour is red or orange.
The tubes were used extensively in electronic test equipment and industrial equipments from the 50s until LED's took over. The latter are still occasionally used, but in turn have given way to low power LCD displays. Another type of display is a high-vacuum type that uses a filament and is to be found in VCRs.
Coupled with the development of displays are the decoders needed to drive them. In particular LED types normally employ segments, which are lit in a particular pattern to display specific numbers. Special decoder chips that convert either binary codes or one of ten address lines to illuminate the required pattern of segments were used for many years, later to be made redundant by including the necessary decoding into processor chips that integrated many other functions. These make use of a data bus that can route codes and commands around the equipment.
Robert Holmes who used to be
kept busy at the Gem Mill works of Ferranti tells me that the
NSP1 was still made there at the tail end of valve production
in 1969 and is a "Strobe lamp" used in things such as
stroboscopes. I guess the modern equivalent of this type, which
must have been used into the 70's, is a flash tube filled with
Xenon. High power lasers are a distant relative of these devices.
I recently read an article in a very old electrical engineering
book about the different effects obtained when applying a high
voltage across a glass tube carrying a soft vacuum of either plain
air, traces of various metallic elements or rare gases. From that
science, which developed through experimentation and trial and
error, sprang fluorescent tubes, voltage stabilisers and presumably
this device. Until the discovery of the electron, near the end
of Queen Victoria's reign, scientists were hard pressed to explain
what they saw. One old chap who was a famous physicist at the
turn of the century hadn't got to grips with new fangled electrons
and was convinced that current flowed through the gutta percha
insulation of undersea telephone cables rather than the metallic
conductor. In terms of exactly where the current flowed he wasn't
too far off the mark. The majority actually flowed in the outer
skin of the conductor and hardly any in the main body of the wire.
Anyone that used to have a good quality wireless
will recognise this type of valve.
This one is on the rare side as its got an old B7 base rather than the more common Octal style.
The top circular area lit up in a rather nice shade of green, something like a newish snooker table baize. With no signal the lit area was about 270 degrees and as a station was tuned in the lower unlit section, which was about 90 degrees started to get smaller until, if the signal was very strong, closed right up.
There probably are very few around now that light up with a decent colour because as the valve gets older the original rich green gets weaker.
I have quite a few of these
things. The end of the valve has an array of little neon elements
which light up in a circular pattern. Its use was in things like
counters where the neons would be externally numbered 0 to 9.
The valve does not have a heater, hence the description "cold".
This type of valve was used
to provide a high current at a high voltage.
This example is an RG1-240A which has an anode rating of 2.2kvolts and can supply a maximum rectified current of quarter of an amp. It's big brother the RG3-1250 is rated at 8000 volts 1.25 amp.
The envelope contains mercury vapour and when operating glows in the typical eerie greenish blue colour of mercury vapour lamps used to light some streets.
Because mercury is considered to be pretty dangerous the valve was rolled up inside it's box in several yards of soft wrapping paper.
Judging from the note which accompanied this example, this type of valve was not an easy thing to use. The circuitry should have a delayed action relay to prevent the HT supply turning on too early and the equipment operator must have had to be pretty intimate with its pattern of use!
