Hi Colin,
About the earth side of things, a good indication of wealth is to measure with a voltmeter between the positive of the battery and the earth side of the lamp while lighted of course. It should be zero volts in an ideal world but I'd say that more than 0.5 V would be an indication of poor earth somewhere. That can be then traced back from the battery plus to other earths.
The same can be done on the minus side evidently.
Sorry if what I said is obvious!
Hope that helps anyway.
About the earth side of things, a good indication of wealth is to measure with a voltmeter between the positive of the battery and the earth side of the lamp while lighted of course. It should be zero volts in an ideal world but I'd say that more than 0.5 V would be an indication of poor earth somewhere. That can be then traced back from the battery plus to other earths.
The same can be done on the minus side evidently.
Sorry if what I said is obvious!
Hope that helps anyway.