When manufacturing resistive coatings that are not in the packaging range of 0.5 to 1.0 Ohms/square some different measuring techniques and precautions may be needed. The old tried and true procedure of applying a large current between two metallic idler rollers and measuring the voltage produced by the resistance of the web between the idlers may lead to some errors. In the case of 0.01 to 0.1 Ohms/Square (rfid and flexible circuitry applications) the necessary currents to get a voltage that is much larger than noise in the wiring system may require currents that can cause some damage to the coating at the contact lines. At the other end of the range, 50 to 5 million Ohms/square (microwave cooking and radar evading and spacecraft charge bleeding and electrostatic loudspeakers) plasmas being used in the coater or even inadvertently produced can "short out" the sample region and cause false readings. The high impedance resistor in the measuring circuit makes the circuit much more susceptible to noise. The average aluminizer boat currents provide a giant jumble of possible magnetically induced noise in the measurement circuits. Some rules of thumb will be presented and some interesting cases discussed. Much material has been extracted from the internet and citations as to where to find the original material are given. Today there are many excellent resources with illustrations available on the internet. The author has reproduced several of them here along with the citation to find the original material. It seems wasteful to try to improve on other's good work. Examples from the author's experience are added in along with opinions as to suitable instrumentation.
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