At the beginning of the nineteenth century, the core of a typical teaching collection was composed by apparatus, which were very similar to the ones proposed in the eighteenth century by 's Gravesande, Nollet, Desaguliers and others lecturer demonstrators and makers. Since 1820 circa, new didactic instruments were introduced. Most of them concerned the fast developing branches of physics such as wave optics, electromagnetisms and acoustics. Instrument makers (and many scientists as well) were extremely prolific in inventing new devices for better demonstrating all the laws of physics and for clearly visualizing all its phenomena. Therefore, around 1900 all the most important German, French and British makers proposed in their thick catalogues thousands of didactic apparatus. But were all these instrument really used? Probably not. Many of them were acquired by schools and universities because they were considered "status symbol" marking the importance and the completeness of a collection. Others were simply shown as "tri-dimensional" illustrations. For various reasons, during the first decade of the twentieth century the number of available didactic instruments was drastically reduced. The introduction of student training laboratory, the increasing cost of labour and of materials after WWI, the needs of a more standardized production, the progresses of physics not only eliminated from the trade catalogues many of the classical but old fashioned instruments but also stimulated the use of modular and simpler didactic apparatus.