Reed relay having conductive bushing and offset current canceling method therewith

#When measuring microcurrent, it is necessary to primarily prevent leakage current from the pattern or the strip line on the measurement path, as well as leakage current from the relay on the measurement path. The reed relay cited in Japanese Patent (Kokai) 2001-14994 that uses a coil bobbin structure with which offset current attributed to heat-induced current and flowing to the reed relay is interrupted and that can be used for measurements on the order of femtoamperes (fA; 10−15 A) is a reed relay for microcurrent. The reed relay in FIG. 5 of JP (Kokai) 2001-14994 is shown in FIG. 6 of the present Specification. The structure of this reed relay will be described. A reed switch 501 is held inside electrostatic shield tube 503 by bushings 502a and 502b, which are made from a strong insulating material, and this electrostatic shield tube 503 is placed in the hollow cylinder part of coil bobbin 504. The area of contact between coil bobbin 504 and electrostatic shield tube 503 is such that space 509 is formed between the inside wall of coil bobbin 504 and the outside wall of electrostatic shield tube 503 by making projections 508a and 508b at the open ends of the hollow part of coil bobbin 504. Thus, joule heat that has been generated by coil 505 that is coiled around coil bobbin 504, virtually is not transmitted to electrostatic shield tube 503. As a result, it is possible to reduce the offset current that flows between relay terminals 517a and 517b and the respective bushings 502a and 502b, while, in the conventional type of the reed relays, the heat induced current is passed between bushings 502a and 502b and electrostatic shield tube 503 by transmitting joule heat to bushing 502a and 502b.

#Nevertheless, developments in measurement technology have led to a need for a reed relay with which the measurement of even smaller currents is possible.

#Japanese Patent No. (Kokai) 8[1996]-279314 relating to dielectric absorption of bushings and Japanese Patent (Kokai) 2[1990]-68829 relating to leakage current and thermo-electromotive force are other technologies for reed relays for microcurrent.

#FIG. 1 is a cross section showing the structure of a reed relay of the present invention (Working Example 1).

#FIG. 2 is a structural diagram showing the structure near the bushing of the reed relay in FIG. 1.

#FIG. 3 is a schematic drawing showing charge transfer near the bushing of the reed relay in FIG. 1.

#According to the reed relay of the present invention, the heat-induced current that flows from the bushing on the outside into the electrostatic sealed tube with heat is recirculated to the conductive bushing by dividing the bushings into inside and outside insulating bushings in a concentric circle and a conductive bushing sandwiched by these insulating bushings with respect to the heat-induced current that flows as a result of the heat that has been transmitted from the coil bobbin to the electrostatic shield tube, and connecting the electrostatic shield tube and conductive bushing with a separate conductive member. Therefore, there is an advantage in that the offset current that flows from the relay terminal to the inside insulating bushing can be reduced even further.

#According to the present invention, an annular first conductive member is placed inside a bushing and further, this conductive member and an electrostatic shield tube are connected by a second conductive member with poor heat conductivity, but good electrical conductivity, such as a lead wire, so that a return path for current is made between the electrostatic shield tube and the bushing and a closed circuit is formed between the electrostatic shield tube and conductive member in order to prevent the heat-induced current that is generated by the joule heat of the coil transmitted from the electrostatic shield tube to the bushing from inducing offset current from the relay terminals. Consequently, charge transfer confined to within this closed circuit and charge transfer between the relay terminals and bushing can be avoided. FIG. 1 shows the preferred embodiment of the reed relay of the present invention, and this is described below as the first embodiment.

#A reed relay 100 of the first embodiment of the reed relay of the present invention shown in FIG. 1 is a reed relay comprising an electrostatic shield tube 103 housing a reed switch 101. Reed switch 101 is mechanically supported inside electrostatic shield tube 103 near both ends inside the tube by two bushings 121a and 121b.