[Al Skierkiewicz]

Tristan,
Mercury wetted contacts have mercury droplets in them that pull together when the contact is closed. They do not form the only conducting surface in the contact but they do make the resistance of the contact low and are especially useful in very low current applications where surface contaminants prevent good contact. These often are in the form of a relay and require specific mounting attitudes to work. These were common in low power microwave applications and in video switching prior to reliable electronic switching.
In mercury contacts, the mercury is used as the contact. Mercury bulbs in thermostats are this type. In these applications, all current flows through the mercury. Such is the case with the rotating device under discussion. The temperature of the mercury is proportional to the current flowing through, much the same as in wire. Arcing within the medium also raises the temperature (significantly) which is true of the switched load used with PWM speed controllers feeding brushed motors.
In comparing the amounts of mercury in these contacts to that in a fluorescent bulb, I believe there is no comparison. The contact will have far more. In the case of the bulb, mostly in a gaseous form.

Bilbo,
The transformers you link are the type that have been used for years on video recorders to link the record and playback heads on rotating disks to the electronics. A good application for these would be sensors and perhaps PWM signals. Please be advised that these carry specific RPM limitations and a finite life. Exceed the RPM and the contacts become intermittent. The brush assemblies are often multiple (3-5) pieces of fine wire in contact with the rotating mechanism. As the RPM increases, the wires become air born and cease to conduct. Low RPM applications are ideal. The lifespan is dependent on the wear of the wires. VTR manufacturers made these assemblies a repairable/replaceable item.