Which feature is often used to define an era

kind of calendar

Explanation:

because an era is a span of time defined for a purpose of chronology or histriography as in the regional eras in the history of given monarchy,a calender era used for a given calendar.

step-by-step explanation:

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(x,y)=(150,300)

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the direction of the magnetic field produced by an electric current will change when the direction of the electric current changes. for example : if a current is flowing in a straight length of wire, the magnetic field is described by a concentric circle (using right-hand rule).