The image below is from the really interesting page Inventing The Interplanetary Probe (linked by @A.Rumlin here).
Above the fluxgate magnetometer there is a pair of short tubes labeled "Variometers". These are not this kind of Variometer. According to the page Remote Scientific Sensors on the same site:
Dolginov designed several types of magnetometers for spacecrafts, including proton precession detectors and variometers. But the most common sensor was the fluxgate magnetometer, which measures a component of the vector field strength in one direction. It is composed of two alternating-current transformers connected in opposite polarity, yielding zero net current. But if an external field is present, nonlinear saturation effects in the iron cores create an imbalance and current is produced. Three perpendicular fluxgates form a triaxial magnetometer, which measures the three-dimensional magnetic vector field.
Dolginov's Magnetometers on Luna-1 to Luna-3 discovered that the Moon has almost no field. Luna-2 data suggested a field of less than 50 nT (its dipole field is today known to be zero). Sputnik-3 had the first spacecraft magnetometer, but it was used primarily to measure the satellite's orientation. His devices were included on virtually all Soviet spacecrafts, including Sputnik-3, Luna-1,2,10, Venera-1,2,3, Zond-3, Mars-1 and later missions. Kosmos-26 and 49 were used to make detailed maps of the Earth's field.
The first planetary probe, Venera-1, had a triaxial fluxgate magnetometer, to measure the field of Venus (if contact had not been lost). It also had two parallel magnetic variometers to measure the magnitude of the faint interplanetary field. Readings from Venera-1 at a distance of 1.9 million km, and from Mars-1 at far greater distances indicated interplanetary fields of 3 to 4 nT. Dolginov's results from the Mars-3 orbiter suggested a weak Martian magnetic field, but recent measurements suggest it is from solar wind, not the planet's core.
Question: How were the Venera variometers able to measure extremely weak, nano-Tesla magnetic fields? Is it actually a specific kind of flux-gate magnetometer or use a different operating principle? Does the diagram show two variometers, or does the pair together comprise a single device?
