Measuring an RSG

What is the easiest way to measuring the break rate of an RSG without
using a tachometer or a strobe light?  The rotary I have is 24
rotating electrodes hooked up for 10 in series at one firing time. 
The gap is set at .040" for each electrode for a total of .400".  In
my arsenal of test equipment I have 3 current probes and an o-scope. 
I'm assuming that you would connect the probe to the lead that is
attached to the movable tap on the primary?  The o-scope I'm using is
a Techtronics RM503 (an old tube type).



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