THE MAGNETOMETER
Mariner carried a magnetometer to measure the magnetic field in interplanetary space and in the vicinity of Venus. Lower sensitivity limit of the instrument was about 5 gamma. A gamma is a unit of magnetic measurement and is equal to 10⁻⁵ or 1/100,000 oersted, or 1/30,000 of the Earth’s magnetic field at the equator. The nails in one of your shoes would probably produce a field of about 1 gamma at a distance of approximately 4 feet.
Housed in a 6- × 3-inch metal cylinder, the instrument consisted of three magnetic core sensors, each aligned on a different axis to read the three magnetic field components and having primary and secondary windings. The presence of a magnetic field altered the current in the secondary winding in proportion to the strength of the field encountered.
The magnetometer was attached near the top of the superstructure, just below the omni-antenna, in order to remove it as far as possible from any spacecraft components having magnetic fields of their own.
An auxiliary coil was wound around each of the instrument’s magnetic sensor cores to compensate for permanent magnetic fields existing in the spacecraft itself. These spacecraft fields were measured at the magnetometer before launch and, in flight, the auxiliary coils carried currents of sufficient strength to cancel out the spacecraft’s magnetic fields.
The magnetometer reported almost continuously on its journey and for 20 days after encounter. During the encounter, observations were made each 20 seconds on each of the three components of the magnetic field.