Hi Hero Thanks also for replying to my query. I did think of using a constant current source but wasn't too sure how to go about it. You mentioned measuring resistances higher than 100m ohms and using an amplifier I can see where you are coming from. When you mention the panel meter would this be a voltmeter and due to the ratio of the amp would give an equivalent resistance measurement? As for powering the tester can I not power it from a mains supply and step the voltage down then rectify it? The problem is I really need 50V d.c. to operate the relay coil also and it would be ideal to have a combined portable tester to perform all the required functions i.e. operate the relay, test the resistance of each contact indivdually and be able to clean the contacts if necessary. Kind Regards Killeavy