An electrical device has a battery, a load, wiring connecting the battery to the load, and a switch in the wiring closable by a magnetic field to energize the load from the battery. The cross-section of the first passage orifice includes a circular main portion, which is provided for receiving an evacuating line, and a lateral bulge, which is provided for receiving a capillary tube. The dynamic self-biasing doesn’t require any switching means. 1 to 10 parts by weight of a lubricant. The solar cell testing apparatus can also include a second electrical probe configured to receive a second voltage at a second location of the solar cell, where the second location is of the same polarity as the first location.