A new instrument has been assembled to measure Thomson scattering from an inductively coupled plasma. Unlike previous designs, the new system employs a fiber-optic array to measure simultaneously up to 12 channels on a Thomson-scattering spectrum. Shutters enable either side of the nominally symmetrical Thomson spectrum to be examined, or permit summing equally spaced channels on opposite sides of the spectrum from the incident laser wavelength. A central channel, use to monitor Rayleigh scattering, enables a simultaneous determination of true gas-kinetic temperatures. Photodetector fatigue is avoided through a rotating mirror that serves as an optical shutter while individual gated integrators attached to each detector minimize the detection of background radiation from the plasma. The new instrument offers high stray-light rejection capability and provides measurements whose precision is photon-limited. Results obtained with the new instrument are presented in a companion paper.