A little Gee Whiz info
I see 0.14 to 0.15 difference between the EGC at my house (right at the service disconnect) and the ground electrode on the pole where the transformer sits, 50' away (that is a 14ga, 50' extension cord)
The .14-.15 change seems to be whether the (120v L/N) washing machine is running. I could hear it cycle.
This is the voltage drop in 50' of service drop.
I suppose I should have looked at the neutral current.