Hi all,
for all those wondering about high value resistors, please remember that electronics is a huge field and no-one can be familiar with them all. Just because you have never seen a high meg resistor doesn't mean they aren't vital components to people working in other fields. High value resistors are often used in integrators, pico-ammeters or electrometers. Pico-ammeters commonly use a resistor in the feedback loop of an amplifier to convert the input current into a voltage. 1 pico-amp X 100 Gigohm = 100 millivolts. Electrometers are used in many research labs, and in electro-chemistry. Pico-ammeters are used with ion chambers for measuring various types of ionoising radiation eg. gamma, X-ray, neutrons, beta particle beams Hig meg resistors, some encapsulated in glass tubes to avoid leakage currents from environmental humidity are quite common in nuclear instrumentation and analytical instruments. An ion chamber contains a filling gas, which may be at high or low pressure and a central electrode. When ionising radiation passes through the chamber, it ionises the gas inside (it can also be air) the high voltage across the tube (can be from 30 to 2000 volts) causes the gas ions to drift and hit the wall and electrode. That gives you a minute current to be measured. For example, ion chambers are used in hospitals with linear accelerators that make high energy X-rays for caner treatment. The linacs aren't highly stable and are usually measured each day to ensure the doctors give a patient enough dose to kill the cancer without burning any more of healthy tissue than necessary. High accuracy pico-ammeter is used with various ion chamber to do this. Ion chambers can be tiny, from 1cc (medical) or big, to 2 litres for nuclear research.
The idea of using a 1.8Meg in parallel with higher values is an inventive solution to a problem caused by available equipment limitations. Being able to find solutions to problems when you are doing field repairs is a valuable skill. So long as you remember that your accuracy will be poorer than the basic instrument accuracy, it will work ok. Another solution is to get ten of the same resistor if quantity is available, parallel them and read their value.
Cheers, Colin