“Mercutronic” is a brand name of Mechanical Enterprises for their mercury-contact switches. The term “Mercutron” was occasionally used, perhaps in error. The series was advertised in Modern Data, April 1970 on page 192 as the “Mercutronic Coding Keyboard” which uses a “mechanical switching approach based on the movement of mercury in a sealed flexible tube.” That issue also mentioned that Mechanical Enterprises’ “Mercutronic Division” would be exhibiting at the 1970 Spring Joint Computer Conference (in booth 49007).
Few details about these switches are known besides what is found in the patents. It seems that at least four series of Mercutronic switches were produced.
All these designs work on the principle of a flexible mercury-filled tube, that is pinched shut by the switch to separate the mercury. The chief objective appears to be fast, bounce-free switching, as they were introduced around the same time as the microprocessor, when keyboards needed special circuitry to deal with contact bounce. As the contacts are sealed, the switches are protected against environmental influences.
US patent 3600537
US patent 3600537 (filed in 1969) is the oldest Mechanical Enterprises Mercutronic patent, for a conductive-liquid snap-action switch. It seems that this was not intended for computer keyboards, as it is shaped like a microswitch.
Behavior Research Methods & Instruments, 1969, Vol. 1 (8) page 329 (PDF page 1) gives a description of these switches under the heading “Subminiature Switches”. They are cited as having the “mechanical advantages of a snap-action switch with the electrical properties of a mercury switch.” They are said to have no bounce, and can switch “60 na” at 24 V DC. (Surely they meant milliamps?!)
US patent 3707611
US patent 3707611 (filed in 1970) depicts a keyboard switch with an external return spring. Wireless World, May 1972 page 251 (PDF page 49) contains an advertisement for “Mercutronic” switches from Tekdata in the UK, with a illustration of a switch similar to that of US patent 3707611, but much lower profile (the base area is drastically shorter). Based on a different advertisement from Tekdata (Electronics Today International, November 1974, page 7), it is clear that these are indeed supplied by Mechanical Enterprises. These switches are rated at 25 million operations, and are noted as being impervious to dust, noise and electrostatic charges. (The unspecified “Mercutron” type in the second advertisement gives a 50 million operation lifetime.)
Modern Data, April 1970, when listing Mechanical Enterprises as attendees of the 1970 Spring Joint Computer Conference, noted that “ME’s Mercutronic Division will display a series of interchangeable-key keyboards to be custom-built for the OEM user.”
Computerworld, Vol. V No. 6 (10th of February 1971) depicts the same swich as shown in Wireless World, in cross-section diagram form, and lists them as model MC-210, suggesting that they are MC series. These fit figure 11 of the patent, where the rubber tube containing the mercury is pinched by pulling it upwards by the hook end of a coil spring.
Sadly the document was scanned in very low quality, but the captions are just about legible. Note that the PDF itself is degraded beyond repair by a defective optimisation process; the image above is from the JPEG 2000 master images, and even that is horribly over-compressed. The illustration below from Electronic Engineer magazine (recreated) is much clearer but omits some of the switch details:
US patent 3845264
An advertisement in ELECTRONICS Australia, November, 1975 depicts Mercutron M-5 series. The design is almost identical to that of T-5 series, and the operating cam mentioned appears to be the same T-bar actuator as found in T-5 switches.
Sadly the PDF went through a defective optimisation process, leaving the image irreparably damaged.