Electrical resistance measurement

DETAILED DESCRIPTION I claim: 1.
A method of monitoring a resistance modifying parameter of a test element by passing a current through said test element and a reference element connected in series therewith, maintaining the resistance modifying parameter of said reference element constant, varying said current to maintain the potential difference across the test element constant, and monitoring the potential difference across the reference element, characterised by varying the current by comparing the potential difference across the test element with a standard potential difference, producing a signal dependent on the difference between said potential differences, feeding said signal to a current modifier in series with the test and reference elements, whereby said current is varied in response to said signal to maintain the potential difference across the test element at a preset value.
2.
A method according to claim 1 including the step of applying a biassing potential difference to the monitored potential difference across the reference element, said biassing potential difference being equal, but opposite in polarity, to the monitored potential difference across the reference element when the resistance modifying parameter of the test element has a predetermined value, whereby the sum of the biassing potential difference and the monitored potential difference across the reference element is indicative of the change in the resistance modifying parameter of the test element from said predetermined value.
3.
A method according to claim 2 in which the biassing potential difference is checked, and adjusted if necessary, by switching the current from the test element to a check element connected in parallel with said test element, comparing the standard potential difference with the potential difference across the check element instead of with the potential difference across the test element to generate the signal to vary the current to maintain the potential difference across the check element equal to the preset constant value, said check element having a resistance equal to that of the test element when the resistance modifying parameter of the test element has said predetermined value, maintaining the resistance modifying parameter of the check element constant, and varying, if necessary, the biassing potential difference until it is equal, but opposite in polarity, to the monitored potential difference across the reference element