The magnetometer instrument consists of two systems: a high-field magnetometer and a low-field magnetometer. Each system contains two identical three-axis magnetometers that measure the intensity and direction of the magnetic field. The low-field system requires isolation from magnetic fields induced by electric circuits in the spacecraft itself. To achieve this isolation, it is mounted on the largest component of Voyager—a 13-meter boom, about as long as the width of a typical city house lot. This boom was coiled tightly in a canister during launch; later, when the package was opened, it uncurled and extended automatically. By using two magnetometers, Ness and his colleagues are able to correct for the residual artificial magnetic fields that reach even 13 meters from the main spacecraft. The dynamic range extends from a maximum field of 20 gauss down to 2 × 10⁻⁸ gauss—a factor of one billion. The fields can be measured as frequently as 17 times per second. The total mass, exclusive of the 13-meter boom, is 5.6 kilograms. [Illustration: Herbert S. Bridge, plasma particle Principal Investigator] Plasma Particles Plasma is the term given to a “gas” of charged particles; the electrons and protons are separate, yet there are equal numbers of each, producing a zero net charge. If the velocities of the electrons and protons are less than about 0.1 percent of the speed of light, they can be measured by the Voyager plasma instrument; if their energies are higher, they are measured by one of the other two particle instruments—the low energy charged particle (LECP) detector or the cosmic ray detector. The plasma instrument, like the magnetometer, was designed to provide basic data on the particles and fields environment of Voyager. The Principal Investigator for the plasma instrument is Herbert S. Bridge of the Massachusetts Institute of Technology in Cambridge, Massachusetts. An experienced space physicist, Bridge has flown similar instruments on many Earth satellites and planetary probes. He also holds the position of Director of the Laboratory for Space Experiments at MIT. On the Voyager experiment, he is joined by one German and ten U.S. Co-Investigators. The objectives of the plasma investigation are directed toward study of both the interplanetary medium and the Jovian magnetosphere. At Jupiter, Bridge expected to determine the plasma populations and processes in the inner and outer regions of the magnetosphere and in the plasma tail that extends beyond Jupiter, much as a comet tail is blown outward by the solar wind. The densities and temperature of electrons were measured, and their origins determined: Some originate near Jupiter and diffuse outward through the magnetosphere; others derive from the solar wind.