Large Magnetic Apparatus for Sliding Electric Contact Investigations

Liquid Metals were used to make sliding electric contacts by Michael Faraday, in 1832. In contrast to solid sliding electric contacts, liquid metals provide uniform coverage to a moving surface and therefore have very low electrical contact losses and are essentially wear free. However, being liquid, the contacts are subject to hydrodynamic instabilities which can cause the liquid to leave the electric contact region and therefore not function. Theoretical considerations suggest these instabilities can become worse at large diameters (1-2 meters). This paper describes an apparatus that was designed to investigate the operation of liquid metal sliding electrical contacts in large magnetic fields. The apparatus consist of a large diameter sliding contact surface rotated by an external drive motor and connected to a direct current power supply of 100,000 amps. The sliding contact apparatus sits in a background magnetic field of up to 2T which can be arbitrarily oriented by changing the current in a superconductive magnet system with a novel dipole-quadrupole configuration. Preliminary data are presented on operation of the test apparatus with liquid metal filled fibers. The Slipring resistance was between 0.1 and 0.2 micro-ohms. Circulating current losses were measured near 3 kW for a 2.2 T radial magnetic field at 100 rpm while losses in 1.6T axial magnetic fields were around a kW at similar speed. The apparatus was operated for several months and no instabilities were observed in the large diameter collector.