Private collection – Gerald Saudan

This unique machine is one of the prototypes of the model 20BTX. It has a special serial number #1004 and was made in December 1960. The 20BTX is specifically designed for statistical and scientific computation. For that reason, it is based on the model BTZG, and has a tens-carry-over which extends to the full capacity of the Product register (Register I).

Some of the features were available as options on other models with the “Z” mechanism, in particular the 20BZS and the 20BTZG. Some others are specific to this model.

The 20BTX incorporates a feature (lever RX) which already existed. This feature allows to maintain the “Repeat Key” during the multiplication and makes it possible to square a number. The machine can square a succession of figures using only the multiplication bar.

The 20BTX has a new “clear key”, marked “IV” which allows to clear the Register IV electrically, so eliminating the old manual lever.

The small lever marked “*” on the picture below, just above the square root symbol can be found also on a standard BTZG (as an option). When pushed to the left, it modifies the operation of the Division key to assist in performing the manual square root algorithm. In particular, it holds rearwards the lever P in order to stop the process of Division after each cycle of iterative substractions related to the Töpler algorithm (sequence of odd numbers).

The real improvement (specific to the 20BTX) is the lever D2. When the lever U is pushed to the right, the lever D2 can be used to transfer directly the content of Register IV to Register I or vice versa.