When I bought a second Leica TCRA 1101 in 2012 I obtained this instrument and a few tripods in the same deal. Accuracy-wise the Wild Heerbrugg T1A is not a particularly special instrument. It is a medium accurate instrument for general work, or as the Virtual Archives of Wild Heerbrugg states it: "...in fact for most survey and engineering tasks where first or second order accuracies are not essential".¹

Originally designed in 1933 as model T1 (so without the 'A') the instrument was the less precise sister of the Wild T2. The instrument was dubbed 'Double Centre Theodolite', probably as it was the first Wild Heerbrugg theodolite to have an exchangeable tribrach (see figure 6), quite similar to the one of the Carl Zeiss ThI Heinrich Wild designed in the early 1920s.

The T1 originally had a similar vial for the vertical circle as the Wild T0. In 1957 it was modified: the vial for the vertical circle was replaced by a liquid compensator (see figure 15) as a result of which it was no longer required to fully level the instrument to obtain a vertical angle within the specs of the instrument.²

The addition of the compensator resulted in the new type name "T1A", where the A stands for "Automatic", followed by the T1A-E - "E" for the "Erect" field of view - in 1959 (the one shown here is a T1A, see adjacent picture and figure 11).² It was the first Wild theodolite - and perhaps the first theodolite at all - to feature an automatic compensator for the vertical circle.

In contrast to the other Wild theodolites produced so far, the Wild T1 was a true repetition theodolite like the Carl Zeiss RThII. For this purpose the instrument has "...two independent cylindrical vertical axes and two horizontal clamps (one each for the lower and upper plate and shaped differently to avoid confusion)..." (see figure 9).¹

The 79mm diameter glass circles of the Wild T1A are read at a single location along the circles and compared to a double line using the micrometer. Both the horizontal and vertical circles are read directly down to 20" by means of the micrometer and estimated to 5" or slightly better (see figure 13 and figure 14).

The automatic compensator for the vertical circle has a working range of 2' and an accuracy of 1". Although the compensator is affected by temperature, with 3" deviation at a combination of 60" instrument tilt and 50°C ambient temperature deviation from the standard temperature of 20°C, this effect is negligible small (the plate level has an accuracy of 30").

Between 1933 and 1970 a total of 51980 Wild T1's have been produced, including all varieties on the model like the T1A, T1E-A etc. In this respect it has been the most productive Wild theodolite for this period and only surpassed in quantity by the Wild levels N1 and N2).⁴

According to the Ahrend Prijscourant 22 of 1962 a "T1-A in metalen stolp, met optisch lood, zonder statief, 400g..." ("T2 in metal cover, with optical plumb, without tripod, 400g") would have cost fl.2,310.-.⁵

This particular Wild T1A is in almost mint condition and came complete with its original container. As with all other Wild theodolites the T1A has been produced as sexagesimal and centesimal instruments. The one showed here is a sexagesimal one.

It is similar in size as the archetype Wild T2 and thus slightly smaller than the later editions of that model. Its telescope has an inverted view with a 30 times magnification, similar to the Wild T2, and has four stadia hairs; two on the vertical cross-hair and two on the horizontal one. The stadia multiplication factor is 100.⁶

Another nice feature for an early 20^th century instrument like this one is the optical plummet. This was a reasonable recent addition to Wild theodolites and implemented in a much better way than on later Wild theodolites. The difference with those later models is that on the T1A the optical plummet forms an integral part with the yoke of the instrument (see adjacent picture). Thanks to this the optical plummet can rotate with the instrument allowing to easily check its proper adjustment. Later theodolites would have the optical plummet in the tribrach and therefore fixated with it in one direction.

The optical plummet can be focussed by pulling it out of the yoke (see figures 7 and 8), while the circular 'cross-hair' (see figure 12) can be focussed by rotating the tube.