Your voltmeter need not have a fuse to protect it, but connecting it to the positive side of a switched circuit (such as the load side of the ignition switch) is a good idea. Otherwise the meter will be 'hot' all the time. Technically, that is not a problem, but a moment of carelessness if you ground the hot side of the meter could cause smoke and damage, but the meter itself would not be damaged in that event. The internal resistance of a voltmeter will prevent it from being damaged by normal voltages within its range of measurement. I don't know the internal resistance of your Stewart Warner instrument, but it is probably several thousand ohms- something like three thousand or so. You can measure its resistance with an ohmmeter- the ohmmeter will show the voltmeter resistance, and the voltmeter will show the voltage applied by the ohmmeter. Start with the ohmmeter on a low range of measurement, then switch to higher ranges until the ohmmeter reads resistance at some level below infinity (open circuit) for the range you have selected. That is, if you want to go through all that, just for curiosity.
Given the measurement accuracy of dash-mounted voltmeters, it doesn't matter much where you make the ground connection, but technically, closer the battery ground is perferred.
Harvester
