A helical spring includes cylindrical portions and a widened portion disposed between the cylindrical portions. The cylindrical portions are disposed at both ends of the helical spring in the axis direction and have a constant outside diameter. In the widened portion, from the both ends of the helical spring toward a center of the helical spring in the axis direction, a diameter of the helical spring increases as a pitch of the helical spring increases. The helical spring can be used as part of a connector, an electrode or an electrometer.
