Test Equipment Anonymous

EC8010, Zenith and I were chatting about fA circuits and electrometers. I mentioned I had 1Tohm resitors (not IT resistors as described in the auction listing), and EC thinks it is worth posting pictures here. I expect he'll post some useful observations.

This is the first time I've opened them. I have considered putting a bag of silica gel in the case to absorb humidity, but there are too many way that wouldn't work.

The resistor markings are positioned to be read by someone looking through the lid, unfortunately. The green resistors appear to be marked "Welwyn 100Gohm 2%" plus a serial number. The other resistors are 1%, but I can't make anything else out. The dR/dV is not small, about 2.5% between 100V and 500V.

1 teraohm is a large resistance, so leakage currents are a serious issue. Water condensing on clean glass wets it to form a leakage film. The earthed conductive paint band prevents leakage current end to end, diverting any leakage harmlessly to earth. Those connectors are MHV, which AI will tell you is incompatible with BNC. I can tell you that people would force BNCs onto MHVs. When you're worried about leakage, absolute cleanliness is a must. I fit BNC caps to all my BNCs to keep the PTFE insulator clean. A cap also slows corrosion of silver plated pins.

I use: https://www.amazon.co.uk/Hoite-Socket-W ... 114&sr=8-2

It's best to ignore the description. There's no way they're waterproof. Sprinkle proof, maybe. More to the point, they should fit the MHV connectors on that resistance box. The Keithley "Low Level Measurements" manual tells you how to clean triax connectors (used for low current measurements), but if you read between the lines, what it really says is, "Don't let them get dirty in the first place." Keep BNCs clean from day 1 and you won't have problems. I put PVC caps over BNC plugs when I'm not using them.

High value resistors are a problem. To achieve a high value from a given material, you make the cross-section as small as possible and the resistor as long as possible. The easiest way to do that is to coat a ceramic cylinder with material and cut a helix from end to end. The smaller the pitch of the helix, the more resistance increases, and resistor manufacturers call this gain. MRS25 metal film resistors are available up to 10M. Above 10M, resistors tend to be thick film, where you put conductive granules in an insulating binder. Conduction is by micro-arcs between granules. Thick film resistors inherently have a negative voltage coefficient and excess noise when a voltage is applied across the resistor. The problems get worse as the ratio of resistance to physical size increases. Those glass bodied resistors have two resistors inside because one is most of the resistance and the other is a trimmer resistor to pull it to value. That tweaking and glass work is why that construction (still available) is so expensive.

I've no experience at all of 1T resistors, but I'd imagine there are very respectable insulators with less resistance. I'd have thought producing meaningful results from them was a considerable exercise in advanced practical physics. Things you wouldn't normally consider, such handling them with your fingers, would certainly be out. You'd want no trace of contamination from the atmosphere. Thermoelectric effects, photovoltaic effects all might impinge.

As for manufacturing them to a guaranteed standard, it must be an elaborate business.

EC8010 is definitely your man when it comes to electrometers and 1T resistors.

I do recall reading about photocells in a valve electronics text book, and the problems caused by the apparently attractive idea of using very high load resistors.

I've just fished out the book. It talks about vacuum photoemissive cells which could be run at up to 200V. The 922 tube produces around 9µA at 0.5 lumens. It says that it would seem that a load resistance of hundreds of MegOhms would produce higher output, but leakage paths across the envelope and insulation cause problems, so the practical limit is around 20MegOhms.

Zenith wrote: ↑Mon Oct 05, 2026 4:20 pm I've no experience at all of 1T resistors, but I'd imagine there are very respectable insulators with less resistance. I'd have thought producing meaningful results from them was a considerable exercise in advanced practical physics. Things you wouldn't normally consider, such handling them with your fingers, would certainly be out. You'd want no trace of contamination from the atmosphere. Thermoelectric effects, photovoltaic effects all might impinge.

Phenolic tag strips are low gigaohm tag to tag.

It is indeed applied physics. Nothing really clever, but things need to be thought about.

I've not had trouble with thermoelectric effects, probably because I've been concerned with currents rather than voltages, but triboelectricity is a problem (charge transfer when a conductor scrapes an insulator). PTFE is triboelectric, so the fatal flaw in an electrometer I saw was a wire supported by the hole in a thin sheet of PTFE. Nice high resistance, but triboelectric and therefore sensitive to vibration and temperature variation (changing temperature would have caused relative movement).

Another PTFE problem is that it is piezoelectric. Mechanical strain causes a movement of charge. Combine that with the fact that there's a phase change at 19C that causes a 1% change in dimensions, if mechanically constrained (think wire in PTFE plugged through a chassis hole), then going through 19C will produce a current pulse. I had that first hand from a chap whose electrometer didn't behave as expected.

Surface leakage currents (rather than bulk) are another problem, and rough surfaces are worse than polished (I'm guessing that they adsorb/trap conductive contaminants.) During the Manhattan Project, PTFE was "flame polished" (melt the surface so that surface tension pulls it smooth). Nobody knew it was carcinogenic. Or maybe they did and figured there was a war on. If using Perspex as an insulator, it's worth polishing all surfaces with metal polish. Wash it thoroughly afterwards.

Clear insulators are better than coloured. Black is worst because it implies carbon, but all colours measure worse than clear.

As an insulator? Never tried, but it's probably quite good.