Dwell is measured by connecting the red lead to the coil (-) terminal, the distributor side, and connect the black lead to ground. A dwell meter is actually a voltmeter, and it's reading an average of the points-open voltage (full charging system voltage) and zero volts when the points are closed. It's expressed in "degrees of dwell" since a set of points that stay closed (dwell together) all the time would be 60 degrees of distributor rotation, and open all the time would be zero degrees of dwell. A 66% dwell reading would be 40 degrees on a 6-cylinder engine.
Jerry

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