Do you know how to differentiate a hygrograph from a hygrometer?

The hygrometer and the hygrograph serve to measure the same thing – humidity – but they are not synonyms. The first records humidity at a specific moment and the second over time. This same distinction is found in many other devices, such as the barometer and the barograph, the thermometer and the thermograph. The incorporation of time as a measurement variable represented a revolution, because it allowed the evolution of several phenomena to be studied. It was especially important in the field of meteorology: continuous records made it possible to identify cycles, compare observations, and better understand and predict the weather. These two instruments are located in a showcase dedicated to Eduard Fontserè at the Faculty of Physics of the University of Barcelona (UB). They are part of this faculty's collection of instruments, which consists of about 600 pieces. There are items related to optics, mechanics, electricity, magnetism, thermodynamics, fluids, astronomy, meteorology... Many of the instruments come from the laboratories and classrooms of the Faculty. Both the displayed hygrometer and hygrograph date from the late 19th century. They have clockwork mechanisms, pens, and cylinders covered with paper. They are the direct ancestors of current data acquisition systems; only the technology has changed. Joan Manel Hernàndez, professor in the department of condensed matter physics and curator of this splendid collection, acts as my host and guide. Pili Mateo, a cultural and heritage technician at the UB, also accompanies us, reminding me of the existence of the UB Virtual Museum, where these pieces and many others are housed. In addition to scientific instruments, the online platform (museuvirtual.ub.edu) contains information and images of works of art, documentary and bibliographic collections, natural science collections, and architectural heritage.The origin of the collection of physics apparatus can be traced to the development of physics cabinets in the 19th century. The designation "physics" in a sense similar to the current one was not established until that century. The first treatises date from the late 1900s, when physics began to be associated with the experimental method. Before that, it was referred to as "natural philosophy". Fontserè is not only honored with this display case, but also with his name on the Faculty's Sala de Graus. He was fundamental in the history of Catalan science, the main driving force behind modern meteorology here, founder of the Meteorological Service of Catalonia in 1921. He also worked in astronomy, seismology, and earth physics, and promoted meteorology based on systematic observation and rigorous data collection. Among his contributions, the Elemental Atlas of Clouds, translated into several languages, stands out. Furthermore, he was one of the main promoters of an astronomical observatory on Tibidabo: the Fabra Observatory. He did so many things! He had time to do them: he lived for 100 years.I have the privilege of going down to a warehouse where there is a string of instruments stored. I notice some rudimentary computers. The screens are cathode ray tubes. They resemble old black and white televisions. Despite their size, they had much lower power and memory than a current mobile phone. "I enjoy playing instruments," reveals Joan Manel. He dedicates himself to cataloging, researching... Practically alone. "Now I have managed to get an intern for two hours a day," he says. The history of instruments is also the history of progress in the ability to measure. Over time they have gained sensitivity, precision, and automation. Today a student can record hundreds of measurements with digital sensors, whereas before they had to read them one by one on a graduated scale and write them down by hand in a notebook.The materials have also changed: before, brass, wood, and glass were used, and often the cables were covered in colored silk. There was a clear intention to make precise and beautiful objects. Today they are much more powerful and precise, but beauty has ceased to be of interest. Plastics, aluminum, and electronics predominate.We leave the warehouse and go to see the star piece: a Grubb telescope, an Irish company, one of themost prestigious in the world for large astronomical telescopes during the 19th and 20th centuries. It is inside and, therefore, does not work. After all, seeing the sky from Barcelona's Diagonal – an area with significant light pollution – is of little interest.

Newton's color wheelOne of the pieces that draw attention in this collection is Newton's color wheel. It is a disc with seven colors: red, orange, yellow, green, turquoise, blue, and purple. The colors of the rainbow. Each occupies the same space, a slice of cake. When it spins rapidly, the different sectors merge and the eye perceives an almost white color. This discovery was one of the great revolutions in physics. Until then, white light was considered the purest form of light, while colors were seen as imperfect or modified lights. Newton demonstrated that it was just the opposite: white light is a mixture of all the colors of the spectrum.