triple-coincidence, and bremsstrahlung above 200 keV in the central tube. When a particle passes through the bundle of tubes, the electronic circuit determines which groups have been penetrated. If a pulse comes from all three groups at once, a triple-coincidence, the particle was a high-energy one, rather than a low-energy one or an X-ray. The triple-coincidence events are telemetered together with the single counts from the center tube to determine counts due to high-energy versus low-energy sources. The high-energy telescope counting rate allows correction of the low-energy telescope data in order that the particle flux incident on the unshielded portion of the low-energy unit can be calculated. Comparing data from the low-energy telescope and the Cosmic-Ray Ionization Chamber (both detect particles in the same energy range) makes it possible to determine the average ionization per particle, from which the type and energy of the particle can be determined. Several magnetic storms occurred during the active life of the experiment. The date of transmission of the last useful information was on 6 October 1959, after which the transmitter failed to operate. Scintillation Counter The scintillation counter experiment was designed to make direct observations of electrons in the Earth's radiation belts with a detector insensitive to bremsstrahlung. This experiment consisted of a cylindrical plastic scintillator cemented to a photomultiplier tube. The instrument viewed space through a foil-covered window in the payload shell, but the instrument also responded to more energetic particles passing through the payload shell. The minimum energies detectable were 200 keV for electrons and 2 MeV for protons. For electrons between 200 and 500 keV, the detector efficiency times the omnidirectional geometric factor was 0.0008-cm2 count per electron; whereas for electrons of energy greater than 500 keV, it was 0.16-cm2 count per electron. For very penetrating particles, the geometrical factor rose to its maximum value of 3.5-cm2. The scintillation counter was sampled continuously for analog transmission and intermittently (every 2 minutes, 15 seconds, or 1.9 seconds, depending upon the satellite bit rate) for digital transmission. The transmitter broadcasting the analog data for this experiment failed on 11 September 1959. Data were received on a limited duty cycle from the digital transmitter until early October 1959. Search-Coil Magnetometer This experiment was designed to survey the gross magnetic field of the Earth, to investigate the interplanetary magnetic field, and to detect evidence of any lunar magnetic field. No interplanetary or lunar magnetic