Mystery Object - Vibron
Tech Night Mystery Object from July 2022
The "mystery object" that turned up in one of the donated boxes of equipment and parts mentioned last month has been formally identified by Lionel, VK7ZLB.
This item took the form of what looked like a vacuum tube with a valve type base at one end, the other end has a ceramic insulated electrode and adjustment ring. Its casing is a lathed stainless steel cylinder about 100mm long and 38mm diameter with quite a hefty weight to go with it. The only visible marking was the word "Vibron" printed on the steel sleeve. The unit is very well made, and looks as if it was very expensive in its day.
Well it turns out not to have been a vacuum tube, but an "Electrically Variable Capacitor", commonly known as a "Vibrating Capacitor", the valve base was probably just convenient as it was used in vacuum tube test equipment.
Internally, the electrode is fixed to a 19mm diameter highly polished gold plated disc 1.5mm thick, with a chamfered edge. There is an identical plate in the bottom of the cavity, and when assembled, there is a spacing of approximately 2mm between the plates. The lower plate is mechanically attached to a shaft which passes through the centre of a non-polarised solenoid coil assembly in the base. The entire unit is also filled with an inert gas to prevent any contamination of the capacitive plate surface.
Depending on the polarity of the applied current to the solenoid coil, the plates move closer together or further apart.
From a found description, "Variation of the gap between the upper fixed plate and the movable lower plate of the capacitor produces variations in capacitance which generate a sinusoidal alternating voltage at 100c/s, of magnitude proportional to the input voltage. The method of operation is to vibrate the lower plate under the control of a non-polarized 6.3 Volt solenoid, which can be energised from a normal heater supply."
"It is the principal component in the input circuits of measuring instruments of the electrometer type and is normally followed by a very stable A.C. amplifier. The device is intended to overcome the limitations inherent in the design of D.C. amplifiers, imposed by the characteristics of valves used in normal electrometer input circuits." It provides an extremely high input impedance.
The device is of particular value in a wide range of specialised "High End" test instruments, from the amplification of minute currents from strain gauges used for submarine cables to the measurement of light intensity of the night sky in conjunction with cadmium photo cells.
The Vibron can measure resistances as high as 10 to the power of 9, Meg Ohms, and currents as small as 10 to the power of minus 10, micro Amps. Yes you did read correctly!
It was used extensively in high end test equipment manufactured by Electronic Instruments Ltd. from Richmond in Surrey England. It is believed they held the patent and were the only manufacturer of this type of device.
A most unusual and I would say pretty rare device to find nowadays.
Thanks to Lionel for his perseverance in solving this one.
There are some excellent pictures and information if you follow the link to Mystery Corner in United Kingdom under Mystery Object #2. This site also has a myriad of other interesting Mystery Objects, a must visit.